Drying defects

The maximum line speed of drying and operating conditions of dryer are often restricted by the onset of defects. The drying related defects could be classified according to the cause of defects - stress induced defects, surface tension driven defects, defects caused by air motion. 

Thickness could be reduced, but the bottom of the coating adhere to the substrate, so the stress is developed in xy plane 

As shown in the Equation 7.3.13, the internal stress depends on the difference of solvent volume fraetion before and after solidification point. Croll s experiment confirms that the internal stress does not depend on the coating thickness until the coating thickness is so large that the net force on the interface exceeds adhesion. At this point stress is relieved by peeling.  

The residual internal stress is a result of combined action of stress and stress relaxation process. Therefore to give a sufficient relaxation time for the coating, we often dry a coated film in a mild operating conditions, e.g., dry at lower temperature and velocity of air. 

Croll illustrated that interfacial work of adhesion could be found by peeling test. 

With solvent evaporation, the coating becomes to be concentrated with polymers, and the coating becomes to have solid like nature (viscoelasticity). Thus the stresses depend not only on the strain rate, but also strain. Strain is deformation Irom the stress-lfee state. After the solidification point, the solvent evaporation continues, so the stresses persist. Croll analyzed the origin of residual internal stress during drying and correlated the internal stress with the coating properties.The residual internal stress, c, for a coating is 

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When attempting to describe more accurately the rheological behaviour of ceramic plastic mixes, one should also take into account the elastic behaviour above the yield point. If a plastic body is abruptly stressed by a constant load, there first occurs rapid clastic deformation followed by delayed elastic deformation and irreversible flow. Similarly, instant as well as delayed relaxation take place after stress relief. If a formed product has only a limited possibility to relax, it retains some interna stress w hich may be the cause of drying defects. 

Large dry dimensions. Timber used in glulam must be seasoned or dried prior to use, so the effects of checking and other drying defects are minimized. Design... 

The spray approach requires the smallest amount of both specimen and stain. While this can be done with a pharmaceutical nebulizer, it is accomplished most easily by using a modified airbrush, such as that described by Tyler and Brandton (103). This device uses a plastic microliter pipet tip to hold the stain specimen mixture and allows the areosol dispersion of as little as 5 il-10 pi of the mixture. The target in this case is a coated grid. Since the droplets of stain/specimen jure so small (5 pl-20 pm in diameter), they dry-down rapidy, so there is less time for any drying defects to occur. When using either the nebulizer or airbrush approach,... 

Coating and Drying Defects Troubleshooting Operating Problems, 2" Edition, Wiley-Interscience, New York, 2006. 

One of the commercially successful hybrid dryers built on the 50 Q. technology is the RF-vacuum dryer for bulk solids such as sawn wood [100], At lower drying temperatures, resulting from reduced boiling point under vacuum, wood retains mechanical strength so it is less susceptible to drying defects such as checking. Because moisture is... 

Gutoff, E. and E. Cohen, Coating and Drying Defects - Tronbleshooting Operating Problems, John Wiley Sons, Inc., 1995. 

Concerning quality issues, no checking or other drying defects were noticed, but sample dimensions used were small. The stuface of the wood samples was light-colored compared with that in conventional drying. The amount of extractives in the solvent phase after treatment was about 0.2% to 0.3% of the wood s dry weight for birch veneer and 0.4% to 0.7% for spruce veneer. 

Gutoff, E.B., Cohen, E.D., and Kheboian, G.I.,, Coating and Drying Defects Troubleshooting Operating Problems, Wiley Interscience, New York, 1995. 

Gutoff, E. B., and E. D. Cohen Coating Drying Defects, Wiley, New York, 1995. 

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