Detachment polymer transfer

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. 

Figure1.8 Deposition of papermakingflneson pulpfiberscoated by cPAM, expressed as the variation in the concentration offines in the supernatant. Owing to polymer transfer the deposition efficiency decreases from 1 to about 0.2, resulting in a net detachment of fines (adapted from Ref [32]).

Polymer transfer is important in papermaking. For instance, if one wants to operate a paper machine in such a way that filler aggregation is absent, it is not sufficient to treat the fiber with a retention aid. Fillers will indeed deposit on such fibers, but filler detachment from fibers will lead to fillers partially coated by polymer, due to transfer. Not all these fillers will be retained in the sheet during papermaking. 

Significant rates of transfer are observed when the initial interfacial adhesion is sufficient to develop cohesive failures in the polymer under the action of the transmitted shear stresses. If this transferred layer is weakly attached to the substrate, it is detached by the same tractions. Providing the geometry of the system allows, this material is displaced from the contact. More film is transferred to the substrate and a high equilibrium wear rate results typically in a polymer pin-on-disc configuration about 10 nm of polymer is removed from the pin during each cycle over the face of the disc. In confined or conforming contacts with... 

Polymer-metal friction pairs are also characterized by frictional transfer of material. This implies the material displacement from the polymer part friction surface onto that of the metal counterbody. Macrotransfer is realized either as a fatigue detachment of polymer particles sticking to the counterbody or as galling, i.e. sticking of the viscous-flow fragments from the polymer surface layer, their extension and movement in the friction direction. 

Several ETFE soft stamps are then transferred on to a tensioned belt, which loops continuously over two rollers. The soft stamp imprints the nanoscale pattern onto the conveying coated substrate (Fig. 13.9). Continuous roU-to-roU imprinting has been demonstrated using a 10 cm wide pattern. The polymer-coated substrate is imprinted and simultaneously UV cured, before the rolling stamp detaches from the imprinted polymer. The UV-curable polymer resist used is low viscosity liquid epoxysilicone [62]. 

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