Tensile bond strength development between liquid-bound pellets during compression

Novel experiments have been performed to measure the tensile bond strength developed between two liquid-bound pellets when they are compressed togethe r at low strain rates. Pellets 20 mm in diameter were made from 75-mum mass -mean sized glass ballotini with water and three different viscosity silico ne oils (0.01, 1 and 60 Pa s). The water-bound pellets formed bonds which w ere brittle and ruptured quickly when strained in tension. The silicone oil -bound pellets were plastic and stretched back a significant fraction of th eir original length before the bond ruptured. The peak tensile strengths an d rapture energies of the bonds were proportional to the radial strain in t he bond zone. A model was developed based on a cold-welding analogy to pred ict the peak tensile strength of the bond as a function of the strength of the bulk pellets and the extent of radial strain in the bond region. There was good agreement between the model predictions and the experimental resul ts for radial strains less than 7%. At higher radial strains, the bond stre ngth appeared to level off, probably because the bonds began to fail by gra dually peeling apart rather than by simultaneous rupture across the whole f ailure plane. This simple model should help in predicting the bond strength formed when two liquid-bound agglomerates collide, which will be important in understanding and modelling granule coalescence growth behaviour. It wa s also observed that the compressive strength of the pellets decreased with increasing liquid viscosity, probably due to a lubrication effect reducing inter-particle friction. This contrasts with the effect of liquid viscosit y seen by other workers at high strain rates, and suggests that the strengt h ranking of formulations with viscous binders may be strain-rate dependent . (C) 2001 Elsevier Science B.V. All rights reserved.