Study On Aging Mechanism Of Polyurethane Material And Its Relationship Between Structure And Properties In Aging Process

In this paper, it has been studied that thermal and oxidative aging mechanism of polyurethane on the quantum chemical theory method from both homolytic bond dissociation energy (BDE) of chemical bond, and unsaturated C=C double bond, also the active site of arfa C-H bond started. All results have been obtained at the (U) B3LYP/6-311++G (2d, p) level by Gaussian 09 program.The main contents and results are as follows:First, it has been studied the reaction mechanism and effect on the mechanical properties of HTPB-IPDI polyurethane thermal aging.In the simplified structure models of HTPB-IPDI polyurethane network, by calculating homolytic BDE of chemical bonds, it has been analysised that the relationship between the energy of chemical bond and thermal decomposition aging. The results showed that C-O bonds attached to CH2 group were the smallest BDE values and prone to fracture decomposition mostly. Based on the thermodynamic calculation results, the feasibility of thermolysis reaction with decomposition products, including of unsaturated cyclohexene, methyl carbamic acid, methylamine and CO2, was validated. The half-life time of such aging process was described according to an exponential decay function of temperature. Molecular dynamic simulations were used to analyze the variation in the network structures and effect on mechanical properties of HTPB-IPDI polyurethane accompanied by the thermal aging. It is found that Young’s modulus (E) and shear modulus (G) gradually reduce; bulk modulus (K) and Poisson’s ratio (v) change weak to strong gradually. The rigidity decreases and the flexibility increases, and mechanical properties will be sharply reduced after aging.Second, it has been theoretical studied the mechanism of the oxidation by polyurethane with O2,·OH and ·OOH.There are six pathways have been discussed of the oxidation of the unsaturated C=C double bond in HTPB polyurethane by O2,-OH and ·OOH. With calculation the activation energy of the oxidation, the result shows that the peroxide P3 only needs the lowest activation energy of 14.03kcal/mol. It has been confirmed that the crosslinking reaction (5) PU+·OH�'IM3�'TS5�'P3 is principal in the oxidation of polyurethane hydroxy butyrate unsaturated C=C double bond.There are two main oxidation mechanisms of the α-H in polyurethane by O2、 ·OOH have been discussed. The activation energy of each path has been analyzed on kinetics. The oxidation (8) of α-H by ·OOH is the control step, of the lowest activation energy of 17.18kcal/mol. It shows that the oxidation to alcohols product P6 by α-H and ·OOH is the main reaction of α-H oxidation in polyurethane. The so called oxidation (8) is:PU+·OOH�'IM7+·OOH�'IM8+·H�'TS9�'P6+·OH, the main principal reaction path.