Stress drying

It has been shown quantitatively (21) that the difference in shrinkage rates between the inside and outside of a drying body indeed results in a tensile drying stress. O. This tensile stress is a function of thickness, F ... 

There is Httle difference between the wet and the dry stress—strain diagrams of hydrophobic fibers, eg, nylon, acryHc, and polyester. Hydrophilic protein fibers and regenerated cellulose exhibit lower tensile moduH on wetting out, that is, the elongations increase and the strengths diminish. Hydrophilic natural ceUulosic fibers, ie, cotton, linen, and ramie, are stronger when wet than when dry. 

Supercritical and Freeze Drying. To eliminate surface tension related drying stresses in fine pore materials such as gels, ware can be heated in an autoclave until the Hquid becomes a supercritical fluid, after which drying can be accompHshed by isothermal depressurization to remove the fluid (45,69,72) (see Supercritical fluid). In materials that are heat sensitive, the ware can be frozen and the frozen Hquid can be removed by sublimation (45,69). 

To maintain a flow of liquid to the surface at a constant rate, the green body must shrink, expelling liquid. Assuming that the flow difiusivity is a constant for simple analysis of this problem. Cooper [18] was able to determine the drying stress at the surface as a flmction of the drying flux ... 

For other shapes the drying stresses are given in Table 14.6. 

TABLE 14.6 Drying Stresses for the Constant Rate Period... 

The Sol-Gel Reaction. The general sol-gel reaction scheme is composed of a series of hydrolysis steps in conjunction with condensation steps (Scheme I). Both the hydrolysis and condensation steps generate low-molecular-weight byproducts. These byproducts often cause serious complications in the preparation of thick crack-free solid materials because of the drying stresses incurred by their removal. 

Drying stresses result from the pressure gradient in the pores of the bulk gel. How they are related to microstructural features and to the drying rate is discussed. 

Fig. 8.10. Schematic illustration of the origin of drying stresses. The length of the slabs in (a) represent the free strains, the uniform lengths in (b) represent the true strain.

Equations for the pressure and drying stress distributions are given in a free gel body dried from both sides by Brinker and Scherer [1] for a number of different conditions. In most of the calculations a uniform porosity and permeability in the gel is assumed. This seems inconsistent with the above-mentioned differential strain (rate). According to Brinker and Scherer density gradients in dried gels are experimentally not observed and so must be small enough to be ignored in a first approximation. 

Consequently the upper part of the crack zone (Fig. 8.11) is free to relax (contract) in response to the compression applied by the liquid, but the gel network ahead of the crack (zone II, Fig. 8.11) is constrained. Large (tensile) stresses occur especially in the zone around the crack tip (zone W). Fracture occurs as soon as the value of surpasses the strength of the gel and this occurs sooner the larger the (effective) crack length is and the larger are the drying stresses (and thus drying rates). 

The maximum stress is obtained after each cycle and so no stress relaxation occurs. Note that the deflection measurements start with a dried membrane which already shows a certain deflection which is equivalent to a tensile stress level of 30 0 MPa. It is not clear at the moment whether it is allowed to sum up these two contributions or that the drying stress relaxes during heating and is replaced by stresses originating in the phase transformation/thermal mismatch processes. In any case when summing up is allowed the final stress in the y-alumina after cooling down is not greater than 30 MPa in the other case it is zero. 

This capillary stress (see Eq. (8.3)) is related to, and usually smaller than, the developing drying stress (see below). The larger the capillary stress the larger the drying stress which reaches its maximum at or just after the critical point (see Eq. (8.7). 

Accelerated schedules for fast drying softwoods - a single step 90/60°C -compared to that offered in Table 8.4a produce evenly dried, stress and strain free wood in two days rather than 4-5 days. The lumber will also be free of kiln brown stain (see later). 

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