Study On Low Infrared Emissivity Polypropylene Composite Materials

Low infrared emissivity materials have wide applications in civil and military fields such as camouflaging military instruments or textiles from infrared detection. Polymer materials are widely used in military textiles, instruments or other defensive domains. In general, due to the presence of organic functional groups, the infrared emissivity of the polymer is high. Polymer-Filler composites have attracted substantial attention because of the potential of combining distinct properties of polymer and filler components within one composite material. To prepare low infrared emissivity materials, it would be desirable to add low infrared emissivity particles into the polymers which have good infrared transparency that might decrease the emissivity of the composite materials.Polypropylene （PP） is suitable for the polymer matrix due to the good infrared transparency and its good performance, easy processing and low cost. The doped oxideds such as Zinc Aluminum Oxide （ZAO）, Indium Tin Oxide （ITO） have potentially application in low emissivity materials because of its special physical , chemical,optical,electrical properties. As we know, metals, either in bulk or thin film, show high spectral reflection and low thermal emissivity. It is possible to obtain low infrared emissivity materials through incorporating metallic nanoparticles or doped oxided into polypropylend. So in this study, Polypropylene was used as a matrix and ZAO （self-prepared）, ITO, and Silver nanoparticles were used as fillers to prepare composite materials by melting blending, and the infrared emissivity of those materials was studied.Zinc Aluminum Oxide （ZAO） particles were synthesized by calcination of precursor prepared by the precipitation method using Zn（Ac）₂·2H₂O and Al（NO₃）₃·9H₂O. The ZAO particles were characterized by X-ray diffraction （XRD）. The results showed that all samples had a single phase with wurtzite structure, indicating that Aluminium atom was put in the crystal lattice of the ZnO. With increase of annealing temperature from 700℃to 900℃, the crystallization of ZAO nanoparticle was improved and the grain size of the ZAO nanoparticles increased. Scan electron microscopy （SEM） was used to investigate the effect of doping and calcination temperature on the morphology of the particles. Al-doping resulted in a decrease in the crystalline quality, and the diameter of the particles distributed from 50nm to 200nm. For nanoparticles, the electrical resistance decreased with Al atom loading and reached minimum at 7at% of the ZAO. To modify the surface of ZAO, aqueous solution of hydrogen peroxide was used to hydroxylate the ZAO powder. The surface of hydroxylated ZAO reacted with KH-570 silane and coupled to form oleophilic layers and then the modified ZAO was analyzed by Fourier transform infrared spectroscopy （FTIR）. Furthermore, the effect of surface modified was verified through a sedimentation experiment in acetone and a dispersion experiment in two phase solvent of water and n-heptane. The pure ZAO could disperse well in water and the modified ZAO could disperse well in n-heptane.Polypropylene （PP）/ZAO nanocomposites were prepared by melt blending by twin-screw extruder, and the bicomponent fibers were prepared by melting spinning from using the master batch. Differential scanning calorimetry （DSC） results indicated that the ZAO was an effective nucleating agent for PP and the melting point of the composites increased with the increase of ZAO particles. Thermo-gravimetric analysis （TGA） results indicated that the thermal decomposition for the different material systems are all one-step processes.The thermal stability and the degradation for polymers were improved by the incorporation of ZAO nanoparticles. The mechanic properties of the bicomponent fibers decreased with the addition of ZAO nanoparticles and increased in the higher ratio of drawing. The sonic orientation of the bicomponent fibers also decreased with the addition of ZAO nanoparticles and increased in the higher ratio of drawwing. The infrared emissivity of the PP/ZAO fabric was higher than the pure PP and the reason was discussed.Low infrared emissivity composites were obtained by blending with PP and silver. TEM results showed that most of the silver nanoparticles were homogeneously dispersed in the PP matrix. DSC results showed that the silver nanoparticle was an effective nucleating agent for PP. TGA results indicated that the degradation of polymers were improved by the incorporation of silver nanoparticles. The rheological measurement showed that the complex viscosity, storage modulus and loss modulus increased with the increase of the silver content. For nanocomposites, yield strength, young’s modulus increased with silver loading and reached maximum at 3-5wt% of the composites, the elongations at break of all composites decrease substantially when compared with neat PP. The infrared （wave length 3-5μm, 8-14μm, 1-22μm） emissivity of PP/silver composites was strongly dependent on silver content and decreased with the increasing of silver content, The electrical conductivity and the dielectric constant increased with increasing silver content, resulting in a decrease of emissivity. The relationship between the electrical properties and infrared emissivity was discussed, and the principle was illustrated by optical theories related to infrared emissivity. The PP/Silver bicomponent fibers were melt spinning using the master batch that been prepared, the infrared emissivity of the textile was measured and the results was analysed. Overall, the results demonstrated that the PP/silver composites as low infrared emissivity material have great potential applications in civil and military fields.