Study of changes in the mechanical and acoustic properties of epoxy resins under long-time polymerization

Changes in the mechanical stress-strain curves and sound velocity of longitudinal waves for epoxy resin ED-20 with different content of polyethylene polyamine (PPA) hardener and diethylene glycol (DEG) plasticizer were studied for cure time from one to 30 days. The obtained dependences of the applied force on the absolute value of stretching for samples with different PPA content and different curing time make it possible to divide the sample state into three types: highly elastic; not fully cured; vitreous fully cured. The parameters characterizing each state are determined. The polymerization process continues with a curing time of 24 – 720 h for all samples. Acoustic measurements were carried out by the pulse method using an automated measuring system. The dependences of the longitudinal sound velocity on the curing time for samples with different contents of PPA and DEG plasticizer are presented. The sound velocity changed much less than the static Young’s modulus within the considered interval of the curing time. The presence of a «kink» in the interval from about 20 to 180 h is a feature of the dependences of the sound velocity on the curing time. The data obtained revealed that the section of rapid growth of the sound velocity corresponds to the transition from the highly elastic state to the glassy solid one. Experimental study of the physical properties of compositions based on ED20 resin with a concentration of the curing agent less than standard provides the possibility of detailed study of the dynamics of polymerization process after the transition from the gel state to the solid state at a long curing time. It is shown that the curing process continues for a rather long time in the solid state. The observed features on the dependences of the sound velocity on the curing time are attributed to the formation of the 3D structure during the transition from the highly elastic state to the solid glassy state. In this case, the value of the longitudinal sound velocity can be an indicator of the degree of completion of the polymerization process of finished products from composite materials based on epoxy resin, for which it may be difficult to measure the mechanical properties.