Home                          Sagging of  a Film as Function of Viscosity and Time

Flow of a coating film after application is an important criteria for the appearance of a coating. Reduction in solvent content (VOC) such as in high solids and water-borne coatings has required to adopt to use of rheology control agents to permit flow and leveling without excessive sagging of the paint film during drying. The sagging speed of a Newtonian liquid can be estimated using as input film thickness, density of the film, viscosity and acceleration due to gravity. Sagging is effected in a coating formulation by the pigmentation, solvent choice,  resin design, crosslinker and catalyst.
Enter data Units     Conversion Sag equation
Gravity acceleration (g)   cm/s2

υ =

pgχ2

   
Density of coating film (p)   g/cm3

   
Film thickness (χ)   cm υ = speed cm/sec
Delta Film thickness   cm p = density g/cm3
Viscosity of film ()   Poise g = acceleration gravity cm/sec2
Delta Viscosity   Poise χ= film thickness, cm
Sagging time   sec   = Poise
Results                      

Film thickness,    cm

Sagging, cm 

                       
Sagging, %

Sagging/Film thickness, %

Paint Testing Manual: Physical and Chemical Examination of Paints, Varnishes, Lacquers, and Colors - STP 500, Gardner Sward, American Society for Testing & Materials, Pub. Date: January 1973, ISBN-13: 9780803100992

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Last edited on:

October 20, 2010

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