Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Eur. Polym. J. Vol. 32, No. 12, pp. 1415-1420, 1996 Copyright 0 1996 ElscvierScience Ltd Pm soo14-3057(%poo79-1 Printed in Great Britain. All rights reserved OOW3057/% $15.00 + 0.00 PHOSPHAZENE-MODIFIED POLYURETHANES: SYNTHESIS, MECHANICAL AND THERMAL CHARACTERISTICS P. RADHAKRISHNAN NAIR,*’ C. P. REGHUNADHAN NAIR’ and D. J. FRANCIS ‘Polymers and Special Chemicals Division, Vikram Sarabhai Space Centre, Trivandrum-695022, India ?Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin-682022, India (Received 27 June 1995; accepted in final form 30 October 1995) AhatraetXyclotriphosphazene-containing trihydroxy and tetrahydroxy compounds were synthesized, characterized and examined as crosslinking agents (CLA) in polyurethane systems constituted independently with polyether polyols such as polyethylene glycol (PEG), polyoxytetramethylene glycol (PGTMG) and polybutadiene polyol (HTPB) and reacted with tolylene diisocyanate. The effect of the nature and concentration of the crosslinking agent on the mechanical and thermal characteristics of the polyurethane network were studied and the results were compared with a system having trimethylol propane (TMP) as crosslinking agent. The tensile strength and glass transition temperature (Tg) increased and elongation de-creased with increasing concentration of the CLA. A relative improvement in these. properties ,was higher for the phosphazene-containing systems vis-a-vis those containing an equivalent amount of conventional CLA such as TMP. Incorporation of phosphazene slightly increased the temperature of onset of decomposition of the PU and decreased the rate of degradation at elevated temperatures. Copyright 0 1996 Elsevier Science Ltd INTRODUCTION Polyurethanes (PUS) are generally characterized by their poor thermal stability due to the thermally labile urethane group. Thermal decomposition starts in the range of 1%200°C. Thermal stability depends on the nature of polyol backbone, the crosslink- ing agent (CLA) and the type of isocyanate used. Several attempts to increase the thermal stability of polyurethanes by introduction of thermally stable groups and CLA are reported [ 1,2]. The introduction of thermally stable moieties in the chain affects to some extent the mechanical properties as well as the processability of the system. The low temperature properties are also affected in some cases. The generally used CLAs for a difunctional polyol include trimethylol propane (TMP), castor oil, etc. In this work we report the use of phosphazene containing tri- and tetrafunctional CLA in various PU systems in an attempt to improve the resulting polymer’s thermal characteristics, as phosphazene is known to impart thermal stability and flame retardant characteristics to polymer systems [3]. Cyclophosphazenes with hydroxyl groups have been reported to be prepared by methyl01 substitution [4]. Polyurethanes containing phosphazenes have been reported [5,6]. This paper concerns the synthesis and characterization of trifunctional (PN-3) and tetra- functional (PN-4) CLAs and a detailed investigation *To whom all correspondence should be addressed. on the mechanical and thermal characteristics of three typical PU network systems (formed in their presence) in comparison to conventionaIly cross- linked PU using TMP as the CLA. The variation in properties at different levels of CLA has also been studied. EXPERIMENTAL M aterials Hexa-chlorocyclotriphosphazene (HCP) was prepared from phosphorous pentachloride and ammonium chloride by a known procedure [71.The crude product was sublimed to separate the trimer. The purity was checked by IR analysis where the peak at 1350 cm-’ due to the tetramer was absent. The polyols poly-oxytetramethylene glycol (POTMG) ex BASF and hydroxyl terminated polybutadiene (HTPB), ex Nocil, India were dried by stirring under vacuum at 6@ -70°C for 3 hr. Polyethylene glycol (PEG) was dried by -tropic distillation with benzene to remove moisture present. Solvents such as chloroform and THF were dried by distillation after adding anhydrous CaCh. 2-Ethyl aminoethanol (2-EAE), ex Fluka and tolylene diisocyanate (TDI), a SO:20 mixture of 2,4 and 2,6 isomers (ex Bayer) were used as received from the supplier. Preparation of crosslinking agent Dry chloroform (100 mL) was added to HCP (34.8 g = 0.1 mol) in a three necked flask. Triethyl amine (30.4 g = 0.3 mol) was added to absorb the HCI liberated during reaction. The system was stirred magnetically and kept cooled at 10°C in an icebath. A solution of 2-ethyl amino ethanol (26.4 g = 0.3 mol) in 25 mL of chloroform 1415