Preparação e caracterização de nanocompósitos poliméricos baseados em poliuretano termorrígido e nanotubos de carbono de paredes múltiplas

Abstract

The main goal of this work is to verify the improvement of thermal and mechanical properties on an elastomeric thermoset polyurethane matrix through the preparation of polymeric nanocomposites with multi-walled carbon nanotubes (unmodified and chemically modified). For that, stable suspensions of nanotubes were prepared (with different concentrations) in poly (tetramethylene ether glycol) with the employment of a roll mill with micrometric separation. It were used carbon nanotubes synthesized at UFMG (Laboratory of Nanomaterials/ Physics Department) that have been modified by acid treatment and by the insertion of isocyanate groups. Images obtained by scanning and transmission electronic microscopies showed that the developed procedures did not cause significant change in the aspect ratio of the nanotubes. Thermogravimetric data indicated that the amount of oxygenated and isocyanate groups placed on the surface of the nanotubes were of the order 7% and 36%, respectively. In a subsequent step, the nanotube/polyol suspensions were reacted with 2,4-toluene diisocyanate and 1,4-butanediol for the production of polyurethane prepolymers. The processing conditions and the quantity of reagents had been developed for keep the free NCO contents in the prepolymers in the range from 7.2 to 7.8. The nanocomposites were prepared by the prepolymer reaction (via free NCO groups) with the 4,4-methylene-bis-ortho-chloroaniline cure agent. Images at different scales obtained by optical, and scanning and transmission electronic microscopies demonstrated that, despite of the existence of millimeter aggregated of nanotubes, the nanofiller was dispersed at the micro and nanometric scales. In all composites there are small regions with aggregated nanotubes, besides of small bundles and isolated nanotubes throughout the matrix. The nanotubes were covered by the matrix, suggesting good adhesion filler/PUE, especially in the composites prepared with acid and isocyanate modified nanotubes. The nanotubes are "sewing" the PUE matrix. This is an important feature for their effect as mechanical reinforcement. Thermogravimetry results showed that MWCNTs dont have (in any concentration or type of nanotube) major influence on the thermal stability and degradation mechanism of the polymeric matrix. The tensile, tear and hardness tests showed that the produced nanocomposites presented significant improvements in all analyzed properties. The best results were verified for the composite prepared with 0.26 wt% of isocyanate-modified nanotubes. In this composite it was verified an increase of 17% in elasticity modulus, and 23% in tear resistance with maintenance of the tensile strength alongation at break, tenacity and hardness. The procedures of modification of nanotubes surface, dispersion in polyol and preparation of the prepolymer developed in this study allowed the production of nanocomposites without major impact on the value chain of the polymer and that can be used in applications requiring large mechanical loads.