Papermaking suspension

Obviously, these fines can interact with other components in the papermaking suspension, such as colloids, polymers and polyelectrolytes. Despite their complicated shapes and polydispersity in size and shape, the homoflocculation of fines subjected to shear follows Smoluchowski kinetics, since flocculation times were found to follow adequately the predictions of Smoluchowski s theory [11]. Hence heteroflocculation is expected to follow these predictions as well. Heteroflocculation between papermaking fines and day, treated with PEI, has been studied experimentally, as was that between MCC (microcrystalline cellulose) and clay [1 Ij. The behavior of these systems is very similar to the heteroflocculation of clay with latex (discussed above). When the clay concentration was below monolayer coverage of clay on fines (or MCC), the fines flocculated, whereas when the clay concentration was above monolayer coverage, the system was stable, that is it consisted of fines, fully coated by clay. 

From the examples presented in this chapter papermaking suspensions are dearly fasdnating complex systems that show a richness of interesting phenomena. Both colloidal and hydrodynamic phenomena play a crudal role. The colloidal interactions can be modified, and thus optimized and controlled, by polymers and poly electrolytes. The time scales of polymer adsorption, partide deposition on fibers, particle detachment polymer transfer, flocculation and break-up of colloidal aggregates determine how a papermaking suspension behaves on a paper machine. These time scales can be controlled by dosage and addition points. Some of the relevant time scales can be predicted by theory, as some of the examples given here show, whereas others require experimental determination, such as polymer transfer rates, particle detachment and floe break-up rates, which are difficult to predict from first principles. Therefore, expensive pilot and mill trials are usually required to optimize and fine-tune the use of additives on a paper machine. Nevertheless, laboratory experiments can provide useful trends and help to eluddate the mechanisms by which additives function. 

Aliphatic acids or esters Mostly high-molecular-weight compounds diethyl phthalate laiiric acid Papermaking wood-pulp suspensions water-based paints food processing... 

Papermaking involves two stages. The first is to break up a raw material to form a suspension of individual fibers, and the second is to form sheets of paper by spreading the suspension on a suitable porous surface through which water can drain. 

Pulping deseribes the processes by whieh wood is redueed to fibre, or strictly to a mixture of fibre and fibre debris. Papermaking, although teehnieally complex is simple in prineiple. Paper is made by spreading a layer of pulp fibres in suspension on the surfaee of a moving wire (mesh) sereen so as to form a wet paper web whieh, after pressing to remove water and eonsolidate the fibre mat, is dried to form paper. 

In the technical process of papermaking a very dilute suspension of fibres is deposited on the moving wire mesh screen of the papermachine (Figure 13.19). A layered structure made of randomly aligned and interwoven fibres is formed by... 

In papermaking systems, the gas in fibre suspensions is generally air (carbon dioxide can also be present depending on the process chemistry and conditions), which can exist in three forms (Matula and Kukkamaki, 2000) ... 

The least problematic of these three forms of air in papermaking fibre suspensions is dissolved air. Because air has limited solubility in water ( 120 mg/1), any dissolved air is in solution and will cause no problons if it remains so. The combination of bound air and free air in a pulp suspension make up the total entrained air. If this entrained air could be viewed under a microscope, very small spheres of air bubbles dispersed into water would be observed (see Fig. 4.1). 

Every pulp and papermaking process has air in its pulp suspension. However, for the types of problems identified in Table 4.1 to occur, an additional component is required. Surface active agents, such as surfactants, must also be present in the system. As mills strive to meet environmental demands, reduce freshwater usage and lower production costs, system closure has resulted in increased levels of detrimental substances and higher stock temperatures. 

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