50 Journal of Research Updates in Polymer Science, 2015, 4, 50-55 E-ISSN: 1929-5995/15 © 2015 Lifescience Global Mechanical and Rheological Properties of Polyurethane Elastomers from Hydroxy-Terminated Polybutadiene and Isophorone Diisocyanate Used as Liners for Composite Propellants J. Quagliano 1,* , V. Wittemberg 1 , J. Gonzalez 2 and A. Bacigalupe 2 1 Instituto de Investigaciones Científicas y Técnicas para la Defensa (CITEDEF), Ave. Juan Bautista de La Salle 4397, B1603ALO, Buenos Aires, Argentina 2 Centro del Caucho (Rubber Centre), Instituto Nacional de Tecnología Industrial (INTI), Ave Gral. Paz/Migueletes, San Martín, Buenos Aires, Argentina Abstract: In this contribution, we studied mechanical and rheological properties of liners based on hydroxy terminated polybutadiene (HTPB) loaded with titanium dioxide and cured with isophorone diisocyanate (IPDI) or toluene diisocyanate (TDI). A thixotropic agent (Thixin R) was added, and viscosity was measured as a function of curing time, leading to much lower viscosities and longer pot life values. Mechanical properties (tensile strength and elongation at break) were not significantly affected but viscosity was reduced at all tested concentrations. Loss modulus (G'') and storage modulus (G') were measured during cure time at three different temperatures (7, 25 and 60°C) with IPDI, and at 7 and 25°C for TDI. Both curves did not intersect during the cure period studied (120 minutes) for IPDI but did so for TDI at 25°C with the formulation having a thixotropic additive. These results suggested that liner formulations will flow if sprayed onto the inner surface of the rocket case before propellant is casted. Keywords: Liner, rheology, applicability, case-bonded, solid rocket motor. 1. INTRODUCTION A liner for composite propellants is basically an elastomeric material applied between the insulation and the propellant to improve interface properties between insulator and propellant and to hold the propellant to the insulated motor case without debonding [1]. The polyurethane-based elastomers on hydroxy-terminated polybutadiene (HTPB) are the polymers of choice when trying to secure a good bond to the propellant rocket motor body and at the same time to protect it from the high temperatures generated by burning the propellant [2]. Viscosity of uncured polyurethane mixtures was reported to increase with the increasing fillers carbon black, silica, aluminum and zirconium dioxide [3]. Excessive viscosity build-up may complicate the application of liners by aspersion. Liners are usually applied by a spatula or spray on the inner face of the motor tube. In order to facilitate the application onto the inner side of tubes is common practice to dilute with dichloromethane in some proportion [4]. However, another approach which excludes the use of solvents is to add a thixotropic agent to increase the fluence of the liner without *Address correspondence to this author at the Instituto de Investigaciones Científicas y Técnicas para la Defensa (CITEDEF), Ave. Juan Bautista de La Salle 4397, B1603ALO, Buenos Aires, Argentina; Tel +54 11 4709 8146; E-mail: jquagliano@citedef.gob.ar affecting its curing properties. Liners are cured with different isocyanates, which differ in its rate of reaction. Aliphatic isocyanates have a slower rate of cure than aromatic isocyanates [5]. Rodi and Petri [6] investigated the effect of the curing agent on the cure time (“pot life”) of different liners compositions and on the properties of these liners for composite propellant formulations. Pot life has been defined as the time necessary for the liner to reach a viscosity of 50,000 cps [7]. In HTPB-based slurry formulations for composite propellants, the flow behaviour was significantly affected by temperature, although these systems have normally a much higher solid filler content than liners [8]. In this respect, fillers have other effects on liner behaviour. When fillers are added to polymeric matrices, normally they increase thermal stability. The properties of the particles themselves (for instance its size, shape and module) can have a significant effect, especially on the mechanical properties, such as specific strain. It was reported that calcium carbonate addition increased the thermal stability of polypropylene (PP) while decreasing yield stress [9]. This is also the case for titanium dioxide in HTPB formulations. However, calcium carbonate was found to have little effect on viscosity build-up in HTPB- TDI polyurethane slurries, other than increasing initial viscosity, allowing adding the filler during the polymerization itself [10]. Mahanta and Pathak [11] pointed out that information of change on viscosity during the curing process is critical in modeling the PU