Bleaching, secondary fibers

Pulp mills. These separate the fibers of wood or other materials, such as rags, Enters, waste-paper, and straw, in order to create pulp. Mills may use chemical, semichemical, or mechanical processes, and may create coproducts such as turpentine and tall oil. Most pulp mills bleach the pulp they produce, and, when wastepaper is converted into secondary fiber, it is deinked. The output of some pulp mills is not used to make paper, but to produce cellulose acetate or to be dissolved and regenerated in the form of viscose fibers or cellophane. 

Deinked secondary fibers are usually bleached in a bleach tower, but may be bleached during the repulping process. Bleach chemicals may be added directly into the pulper. The following are examples of chemicals used to bleach deinked secondary fibers hypochlorite [HCIO, NaOCl, Ca(OCl)2], hydrogen peroxide (H202), and hydrosulphite (NaHS03). 

Secondary explosives, 10 722, 724 Secondary fibers, bleaching of, 21 51-52 Secondary fixed points, Tgo values for,... 

In recent years, less chlorine and more sodium hydroxide have been used for bleaching. At present, chemical pulps and the deinking of secondary fibers are the primary users of chlorine bleaching techniques. The European paper and pulp industry uses ozone, oxygen, and peroxide in place of chlorine. 

With bleaching in stock preparation systems the optical properties of secondary fibers are improved the brightness of the stock is increased and a possible color shade is reduced. 

Economic reductive bleaching of secondary fibers requires an oxygen-free stock as dithionite is sensitive to the oxygen contained in the air. Therefore the stock has to be deaerated which can be done sufficiently at a medium consistency of about 10 to 15 %. FAS is less sensitive, thus operating consistency can be as high as 30% and a Disperger may be used for admixing of FAS. Best results are obtained when... 

Dispersing is used in secondary fiber processing. It is located at the point of water loop separation where the suspension is dewatered up to a consistency of about 25 to 35 %. Often bleaching is done in combination with dispersing. For high quality demands two dispersing steps may be applied in the process. 

In spin dyed secondary acetate threads, fibers, and films, P.B1.25 exhibits good textile fastness properties the only problem is a certain lack of fastness to bleaching with sodium hypochlorite (Sec. 1.6.2.4). Its fastness to light in 0.1% spin dyed specimens equals step 3-4 on the Blue Scale, while 1% samples equal step 5. 

The most commonly used EWAs in laundry products today are shown in Table 28.2 and represent three chemistries—distyrylbiphenyl, coumarin, and stilbene. The selection of the EWA to be used in a specific type of laundry product will depend on several factors such as compatibility with the formulation, fabrics, product claims, laundry conditions, application, and manufacturing limitations. For example, compounds 60 and 62-65 are substantive to cellulosics and compound 61 is substantive to sUk, wool, nylon, secondary acetate, and triacetate fibers. Eor bleach-based products (e.g., hydrogen peroxide) compound 60 is used as distyrylbiphenyl chemistry exhibits the required stability. 

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