Avoiding "mud" cracks during drying of thin films from aqueous colloidal suspensions.

The critical cracking thickness of films obtained by drying aqueous alumina suspensions has been investigated. The effects of solution chemistry, binder and binder crosslinking were studied. Films formed from flocculated and dispersed suspensions are compared. The influence of the addition of the polymeric binder, poly(vinyl alcohol) (PVA) was also investigated. In addition, in some of the dispersed suspensions the PVA was covalently crosslinked. The critical cracking thickness is found to be 3 times greater for the films obtained from dispersed suspensions than for the films obtained from flocculated suspensions. The superior mechanical properties are primarily due to the higher final solids concentration in the films obtained from dispersed suspensions. Addition of PVA leads to an increase of the critical cracking thickness by a factor of two for both dispersed and flocculated systems. When the PVA is crosslinked, the mechanical properties of the gel during drying are improved and the critical cracking thickness is increased 10 fold with respect to the suspensions with uncrosslinked PVA.