Two coating matrix materials

polyurethane and silicone rubber were used for investigating the relationship between absorption and molecular structure of polymers under hydraulic pressure. The polyurethane was synthesized with toluene diisocyanate and polyethylene glycol as main raw materials

and the silicone rubber was prepared with vinyl polydimethyl siloxane and hydrogen-containingsilicone fluid. The molecular structure of polymers was analyzed by using Fourier transform infrared spectroscopy. The sound absorption performance was measured by the standing wave tube method. The mechanical performance of coating films was characterized by tensile and shear moduli measurements

and their volume change under hydraulic pressure was simulated by using a file ANSYS 8.0. The different polymer presented different sound absorption performance under ordinary and hydraulic pressure. As frequency ranged from 3 to 30 kHz under ordinary pressure

the average absorption coefficient of silicone rubber-based coatings was 92.6%

and that of polyurethane-based coatings was 68.5%. The sound absorption performance of coatings changed noticeably with hydraulic pressure increased. The sound absorption performance of silicone rubber with flexible molecule chain structure decreased rapidly when the hydraulic pressure increased. The average absorption coefficient changed from 26.9% under 0.5 MPa down to 13% under 3 MPa. The sound absorption ability of polyurethane with rigid molecule chain structure increased when the hydraulic pressure increased. The average absorption coefficient changed from 75.2% under 0.5 MPa to 87.6% under 3 MPa. The results of finite element analysis showed that the volume compressibility of silicone rubber-based coatings increased from 0.55% to 3.29%

and that of polyurethane-based coatings changed from 0.03% to 0.20% when the hydraulic pressure increased from 0.5 MPa to 3 MPa. The polymer of polyurethane-based coatings was a polyether-type polyurethane with a large number of intermolecular hydrogen bonds and carbonate polar groups from TDI and hydroxyl groups. The cohesive power of the carbonate groups was very large

the total power of interaction of the rigid molecule chains with large numbers of carbonate groups was powerful

which engendered network-like structure among linear chains. The free volume of polyurethane with rigid molecule chains changed slightly as the volume compressibility was less under hydraulic pressure. The flexible molecule chains of silicone rubber were easy to slide under hydraulic pressure

which resulted in rapid decrease of free volume

and the molecular motion was blocked. The sound absorption performance was affected by the size free volume and the movement of polymer molecules. These results indicate the importance of molecular structure and free volume adjusting in preparing excellent anechoic coatings under high hydraulic pressure.