Water drop penetration time

The absorbency was found to be decreased with the reduction in particle size (Fig. 19.42]. This may be elucidated by the example of lotus leaf effect. Distribution of nano chitosan particles as a thin layer over and beneath the surface may roll out the water droplets. Nevertheless, the absorbency of nano chitosan-treated samples is still within the tolerable limits of conventional wet processing conditions since this rise in water drop penetration time is due to the initial resistance offered by nano chitosan particles and not due to the hydrophobicity. 

Recent experiments by the authors studied the water repellence of PET fabrics (technical fabrics), photochemically treated in the presence of, e.g., 1,5-hexadiene, 1,7-octadiene, diallylphthalate (DAP) and l//,l//,2//,2H-perfluorodecyl acrylate (PFDA). Exemplary experimental data are summarized in Fig. 13 showing drop penetration times in excess of 1 hour (measurements were stopped after this time) and DuPont grading of up to 8. The relevant values for the untreated fabrics were drop penetration time approx. 20 s and a DuPont grading 0. Based on the well-known effect of heat treatments on long-chain fluoro compounds (cf. Sections 4 and 5.1), the samples treated with PFDA were also characterized following a further heat treatment. As was found in the case of wet-chemical finishes and plasma-deposited fluorocarbon thin layers, the water repellence of the samples could be further enhanced by heat treatment in this case also. 

Drop penetration times of water droplets (in fact an aqueous dyestuff solution) dropped onto the technical PET fabric are summarized in Table 13.3, the drop penetration characterized according to the TEGEWA procedure. 

Table 13.3 Water wetting behavior of technical PET fabrics following (a) laser-roughening, (b) photo-chemical modifications using different reactive media, and (c) combined laser roughening and photo-chemical modification. Both laser and photo-chemical modifications were performed on both faces of the fabrics. Water wettability was characterized by measurement of the drop penetration time of an aqueous dyestuff solution according to the TEGEWA procedure (see text).

Table 13.5 Effect of carbon-rich debris produced by the laser irradiation of p-aramid fabrics at 248 nm on their water wetting behavior. Data give the drop penetration time of an aqueous dyestuff solution measured according to the TEGEWA procedure (see text). For cleaning, the samples were extracted for 4 hours in water/methanol and further 4 hours in petroleum ether.

U. Single water drop in air, liquid side coefficient / jy l/2 ki = 2 ), short contact times / J 1 lcontact times dp [T] Use arithmetic concentration difference. Penetration theory, t = contact time of drop. Gives plot for k a also. Air-water system. [lll]p.. 389... 

Cone. Absorbency Time taken for water drop to penetrate, seconds ... 

Various conclusions can be drawn from the table. With a slow drop penetration (elapsed time 80 s), the untreated fabric can be considered only modestly hydrophobic. One has to take into account that (a) the water contact angle on untreated PET film is of the order of 72 and (b) that capillary effects can be expected to be significant in the observed behavior. 

The drop penetration test was developed by Sookne et al. [168]. The apparatus consists of a drop-forming device with capillaries mounted at the bottom of a container, a tube to shield the spray from draft, and a sample holder designed to collect the water penetrating the fabric. The time needed to collect 10 mL of water is measured. 

Alcohol holdout tests, which are also used to measure aqueous fluid repeUency, iavolve placing drops of aqueous isopropyl alcohol solutions of concentrations 10, 20,. .. 100 wt % on a fabric surface. The rating for the fabric is based on the most concentrated solution that does not penetrate the fabric ia the specified time frame (3M Water RepeUency Test II, Water/Alcohol Drop Test (41) INDA Standard Test 80.6). 

Soil Column Tests. In the sand penetration test, a minimal amount of water was used. No consideration was given to the hydrostatic pressure which would occur in nature from a body of surface water. A new soil infiltration test was developed to take this into consideration. This test used a maximum amount of water (200 mL) on a minimum amount of treated soil (10 g) and was restricted only by the dimensions of the laboratory equipment. Our aim was to prepare an hydrophobe for soil which would support water over an extended period of time. Whereas water passed through soil treated with hydrophilic compounds within 8 hr, 2 weeks or more were required for penetration through an hydrophobe-treated soil. In the latter case the water level dropped 6 mm or less each day, showing that the cationic surfactant greatly hindered, but did not completely restrict the passage of water. The tests were usually terminated after 2 weeks, due to the large number of samples to be tested. 

Alcohols also promote wettability and penetration of the wood surface. This may easily be shown by the following simple experiment. When equal sized drops of distilled water were placed on the surface of a freshly planed piece of southern yellow pine, the times for the drops to completely soak into the wood were observed. On the early wood it took 65 seconds and on the latewood 179 seconds. When similar drops of 50% ethanol solution were used instead of pure water, it required only six seconds to disappear into the earlywood and 26 seconds into the latewood. However, if a small drop of adhesive syrup, with no hardener added, was placed on the wood surface, no adsorption took place at all. It was surmised that the viscosity prevented its permeation. When the adhesive was diluted with 50% alcohol it was readily absorbed and produced a red stained spot on either earlywood or latewood areas. This showed that the low molecular weight adhesive molecules could readily permeate the wood structure before condensation with the curing agent. 

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