Penetration into Paper

The penetration of fluids into paper is also a very important material property for many product types. It is influenced by a number of factors, not least of which is the sheet structure and porosity. In 

This wetting process may be described in terms of a balance of specific surface energies —the Young equation  

Clearly, if the contact angle between the solid and the liquid is 

Retardation of the rate of penetration is necessary for many products and this can be brought about by the creation of a low energy, hydrophobic surface at the fibre-water interface which increases the contact angle formed between the drop of liquid and the surface. This important change can be achieved chemically in the process known as sizing which is discussed more fully in Chapter 7. 

If the fibres in the paper have been rendered hydrophobic by sizing, but the sheet has an open structure and there has been no surface treatment to cover the sized fibres, then the web will show a high contact angle. However, if the same web is tested by a penetration-type test, the sizing level will be low. 

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Fluidyibsorbamy. Fluids like ink penetrate into paper during the printing process. The further the ink penetrates, the less glossy the print. The degree of penetration in paper is generally a function of the paper porosity and wettabiUty by the fluid. It can be controlled by the particle size, shape, and chemical nature of the filler or filler surface. In particular, plate-like fillers, such as clays, tend to produce the best fluid holdout because they tend to overlap and reduce the porosity at the paper surface (see Inks). 

Absorption. Some inks (eg, oil-based newspaper inks) dry by penetration or absorption into the pores of the printed stock, which has a blotter or sponge effect. This is accompHshed by the gross penetration of the ink vehicle into the pores of the substrate, the partial separation of the vehicle from the pigment, and the diffusion of the vehicle throughout the paper. The abiHty of an ink to penetrate into paper depends on the number and size of the air spaces present in the paper, the affinity or receptivity of the stock for the ink, and the mobiHty of the ink. 

As water penetrates into paper, it swells fibers, and also disrupts hydrogen bonds that hold paper fibers together. Over a short ( 0.1-10 sec) timeframe, this can lead to paper cockle, a localized warping of the original flat sheet (see Fig. 6). The simple way to fight cockle is to put less water on the sheet. Over longer (1 minute... 

Liquid toners penetrate into paper fibers and appear as a thin coating through which paper fibers can be seen quite unmistakably. Liquid toner copies can sometimes be mistaken for documents produced by other forms of printing (for example, an ink jet printer or an inked stamp). Microscopic examinations, at a magnification of 100 x, can also discriminate the toner s methods of fusion with the paper substrate. 

Stamp-Pad Inks. These inks are impregnated into a cloth or foam mbber pad and transferred by pressure to mbber type which is then stamped or impressed against the substrate. The inks must be completely nondrying in the pad and yet dry by rapid penetration into the paper. Since it is desirable that the total ink soak into the stock, dyes are used rather than pigments. The vehicles used are usually glycols. 

Adhesives for paper tubes, paperboard, cormgated paperboard, and laminated fiber board are made from dispersions of clays suspended with fully hydrolyzed poly(vinyl alcohol). Addition of boric acid improves wet tack and reduces penetration into porous surfaces (312,313). The tackified grades have higher solution viscosity than unmodified PVA and must be maintained at pH 4.6—4.9 for optimum wet adhesion. 

Surface-type filter media are distinguished by the fact that the solid particles of suspension on separation are mostly retained on the medium s surface. That is, particles do not penetrate into the pores. Common examples of this type of media are filter paper, filter cloths, and wire mesh. 

Although many papers have been published on the virucidal (viricidal) activity of biocides there is little information available about the uptake of bioeides and their penetration into viruses of different types, or of their interaction with viral protein and nucleic acid. 

In view of the immense commercial importance of phthalocyanines as pigments, it is perhaps surprising that only a few are of importance as textile dyes. This is primarily due to the size of the molecules they are too large to allow penetration into many fibres, especially the synthetic fibres polyester and polyacrylonitrile. An example of a phthalocyanine dye which may be used to dye cellulosic substrates such as cotton and paper is C. I. Direct Blue 86 (96), a disulfonated copper phthalocyanine. In addition, a few blue reactive dyes for cotton incorporate the copper phthalocyanine system as the chromophoric unit (Chapter 8). 

In fruit penetration studies 8 pounds of fruit were first thoroughly scrubbed with warm 10% trisodium phosphate solution and then rinsed thoroughly with distilled water. Citrus fruits, if depth of penetration into the peel was of interest, were peeled in longitudinal sections with a buttonhook peeler and the albedo or white portion was separated from the flavedo or colored portion. The separated peel was placed in pie tins lined with waxed paper and dried in a forced draft oven at 65° C. for 16 hours. The dried peel was then crushed and steeped for 48 hours in a measured volume of benzene sufficient to cover the sample. If, on the other hand, only the total amount of DDT in the peel was of interest, the fruit was halved and juiced on a power juicer. The pulp was removed, the peel sliced, and the sample dried and treated as before. Thin-skinned fruits, such as apples, pears, and avocados, were peeled with a vegetable peeler, cores or seeds were removed, and the pulp was sliced in thin slices. Pulp and peel were then dried and treated in the same way as the citrus peel. The steeping completed, the samples were filtered through Sharkskin filter paper and the volume of benzene recovered was noted. 

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is used to obtain spectra of powders and rough polymeric surfaces such as textiles and paper. IR radiation is focused onto the surface of the sample in a cup resulting in both specular reflectance (which directly reflects off the surface having equal angles of incidence and reflectance) and diffuse reflectance (which penetrates into the sample subsequently scattering in all angles). Special mirrors allow the specular reflectance to be minimized. 

Almost none of the cited papers dealing with water absorption by syntactic foams deals with the mechanism by which water or other small molecules penetrate into these materials. Filyanov et al.148,149) undertook one of the first attempts using the ED-20 epoxy oligomer—glass microsphere system. Water absorption by a filled polymer is known to depend on the sorptive properties of the binder, the stability of... 

Lin and Yang (1987) also calculated the thermodynamic parameters of diazepam for micellar solubilization in Pluronic surfactant solutions at different temperatures (Table 13.4). For all systems, AG was negative, indicating micellar solubilization was spontaneous. The sign of entropy has been associated with the location of solubilized molecules within the micelles. Positive values have been observed for molecules embedded in the micelle center and negative values for adsorption of the molecules on the micelle surface. The results in this paper indicate that in the F-108 and F-88 Pluronics, diazepam molecules can penetrate into the micelle interior, whereas for F-68 and lower concentrations of F-88, diazepam is adsorbed on the micelle surface without penetration into the micellar core. 

Another method presented in this paper is the indirect eb method when the C -lace of a LiNbOs ferroelectric is preliminary coated by a highly defective layer of the amorphous photo-resist material pmma. The thickness of this dielectric layer is large enough to protect the LiNb03 from penetration of high energy electrons into the bulk. In the presented calculations and simulation a very limited number of electrons penetrated into the LiNbOs crystal, so most of the injected electron charge remains trapped in the pmma layer. 

One of the major features of this method is the rapidity of penetration of chemicals into paper and the extreme rapidity of drying the paper even at room temperature. This can avoid the problems of drying and crocking. 

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