Welding Line Structure And Strength Of Polymer Blend And Morphology Development In Presence Of Ultrasonic Oscillation

The mechanism of decrease of weld line strength of polymer was investigated. Ultrasonic oscillations were introduced to improve weld line strength. The mechanism of ultrasonic improvement of weld line strength was studied.The V-notch on the surface of sample and poor bond layer act as stress concentrator, resulting in the decrease of weld line strength of PS. The form of transcrystallization between the weld line interfaces and high crystallinity causes the weld line strength of the skin of PP close to the bulk strength, resulting in the weld line strength of PP close to the bulk strength.For the blends that the dispersed phase shows fibrous morphology in weld line region, the decrease of weld line strength is due to the dispersed domains highly oriented along weld line, which acts as the stress concentrator, causing the dramatic decrease of weld line strength. The incorporation of compatibilizer results in the disappearance of oriented dispersed phase, which is beneficial to the increase of weld line strength. The change of viscosity ratio of matrix and dispersed phase affects the morphology of dispersed phase in weld line region greatly. The change of viscosity ratio of dispersed phase and matrix from over one to below one induces the morphology of dispersed phase variety from fibrous shape to spherical shape in weld line region. For the blend that the dispersed phase shows flaky morphology in weld line region, the dramatic decrease of weld line strength is caused by flake-dispersed phase, which holds up the diffusion of matrix between the interfaces of weld line. The incorporation of compatibilizer results in the disappearance of flake-dispersed phase, promoting the interdiffusion of matrix molecular in weld line, causing theincrease of weld line strength. The change of viscosity ratio of dispersed phase and matrix from below one to over one induces the morphology of dispersed phase variety from fibrous shape to spherical shape in weld line region. For the blend that dispersed phase shows spherical shape in weld line, the incorporation of compatibilizer is beneficial to the increase of weld line strength.Among PS, PP and their blends, the effect of injection parameters on the weld line strength of PS is more than that of others. Compared with other injection parameters, the melt temperature and mold temperature are most available to improvement of the weld line strength.The presence of ultrasonic oscillations can enhance the weld line strength of PS. Compared with mode I (ultrasonic oscillations were induced into mold at the whole process of injection molding), the induced ultrasonic oscillations as mode II (ultrasonic oscillations were induced into mold after injection mold filling) is more effective to increase weld line strength of PS. The mechanism for ultrasonic improvement of weld line strength is that ultrasonic oscillations can improve the molecular diffusion across weld line of the melt at the core, which is benefit to the increase of weld line strength, and make against the fusion of melt at the skin, which is disadvantageous to the increase of weld line strength. So the induced ultrasonic oscillations as mode II means that the molecular chains at weld line can get primary diffusion before ultrasonic oscillations are induced, resulting in the stronger bond strength of melt at the skin to resist the ultrasonic oscillations than that of mode I, which is beneficial to the increase of weld line strength. The presence of ultrasonic oscillations causes the diminishment of the crystal size and promotes the interfusion of molecules at the core of PP in weld line region, resulting in the increase of weld line strength slightly.For the blend that the dispersed phase shows fibrous morphology in weld line region, the presence of ultrasonic oscillations can reduce the degree of orientation of dispersed phase near the weld line, causing the breaking site of the specimens inside weld line, resulting in the increase of weld line strength. The faint effect of ultrasonic oscillations on the weld line strength of blend with flaky morphology of dispersedphase in weld line is due to the flake shape of dispersed phase in the weld line, which holds up the diffusion of molecules in the weld line interface. For the blend that dispersed phase is spherical in weld line, the presence of ultrasonic oscillations can improve weld line strength. The mechanism is that ultrasonic oscillations can promote the molecular diffusion across weld line of the melt at the core, resulting in the increase of weld line strength.For PP/HDPE (80/20) , the crystallinic polymer blends, ultrasonic oscillations would reduce weld line strength of skin, promote the increase of the crystallinity of PP and diminish the crystal size of core in weld line, which is beneficial to the improve of weld line strength of core, resulting in the increase of weld line strength slightly in the presence of ultrasonic oscillations.