Rheological Properties and Printabilities of Polybutadiene/Carbon Black Ink

It is important to study the rheological properites of emulsified ink to obtain good printability and excellent printing quality in offset lithography. In this study, the effects of the molecular weight of a disperse medium on viscoelastic properties and the printabilities of offset lithographic ink were mainly discussed. The suspensions of carbon black in polybutadiene were used for the sample inks. The printability of the offset ink was investigated from the viewpoint of the dynamics of ink splitting and transfer in a roll nip. The growth and the rupture of ink filaments between rollers were observed using a video system which consists of a microscope, a CCD video camera, and a personal computer with an image-processing board. The acoustic study of the ink splitting was performed using a sound-analyzing system consisting of a microphone with sensitivity at higher frequencies and an FFT analyzer. The power density of the ink-splitting sound at the frequency region between 5 and 30 kHz decreases with increasing water content in ink. The length of ink filaments, the quantity of transferred ink, and the ink tack decrease with increasing emulsification. The dynamic viscoelastic properties and the yield value increase with proceeding emulsification. These phenomena can be explained by the alteration in viscoelastic mechanisms in connection with the molecular weight of a disperse medium.