Grinding, mechanical degradation

Crushing. Byrskii et al. (8) used a vibration ball mill to study the volatile compounds released from a series of crushed polymers. They chose this method to intensify the mechanical degradation process and thus to increase the rate of volatile evolution from the polymer. For polymers such as polyethylene (PE) and polytetrafluoroethylene (FIFE), compound evolution rates are low from other methods of mechanical deformation hence, it is difficult to obtain mass spectra. Byl skii et al. used this technique to successfully obtain mass spectra of compounds from PE and PTFE and demonstrated the feasibility of the vibration ball mill for performing kinetic studies of mechanical degradation as a function of the amplitude of the vibration and duration of the grinding. 

Most past research on grinding mechanisms for ceramics has followed either the indentation fracture mechanics" approach or machining" approach. The indentation fracture mechanics approach would seem to offer the possibility of describing both the material removal process and its influence on strength degradation in terms of the force or depth of cut at an... 

Malkin, S. and Ritter, J.E., Grinding mechanisms and strength degradation for ceramics. Intersociety Symposium on Machining of Advanced Ceramic Materials and Components, ASME, 1988, pp. 57-72. 

Grinding and milling at low temperatures as well as ultrasonic investigations in solution show that mechanical degradation can be compared to the mechanism of statistical chain cleavage at temperatures near 250 C. 

Mechanical degradation comprises both physical fracture phenomena and chemical changes induced by mechanical stresses. It is met in operations such as stirring (in solutions or other states), grinding, milling. 

Churchman GJ, Gates WP, Theng BKG, Yuan G (2006) Clays and clay minerals for pollution control. In Bergaya F, Theng BKG, Lagaly G (eds) Handbook of clay science. Elsevier, Amsterdam, pp 625-676 Cicel B, Kranz G (1981) Mechanism of monmorillonite structure degradation by percussive grinding. Clay Miner 16 151-162... 

Grinding or milling causes degradation of many polymers. The process of mastication of natural rubber involves a mechanically initiated, autoxidative degradation which lowers the molecular weight to a level where the material is easier to process on a commercial scale. 

Mechanochemical Degradation. Mechanochemical degradation occurs in polymers as the result of an applied mechanical force. This type of degradation is quite common in machining processes such as grinding, ball milling, and mastication. 

Thus, macroradicals have been obtained by stretching fibers (20), deforming plastics by compression (37), ball mill grinding (11), freezing and grinding of polymer solutions (10), ultrasonic irradiation (I), mastication (19), dispersion in a microblender (25), and other mechanical techniques (36). Many reviews on the formation of macroradicals by degradative processes have also been published (5, 12,13,16, 33). 

Pectins are also readily degraded by oxidation (Neukom, 1963), y-irradiation (Sjoberg, 1987), mechanical treatments such as grinding and extrusion (Bock et al., 1977 Ralet and Thibault, 1994), and dehydration (Ben-Shalom et al., 1992). 

To meet automotive demand for parts with high gloss Class A finish, ERCOM has to resort to heavy grinding of the recovered material, which is expensive and degrades the length of fiber to a point where mechanical properties are so poor as to be indistinguishable from cheap fillers. The cascade effect is therefore in play, rather than the closed loop. ERCOM notes that the most valuable component of recovered SMC is, in fact, the glass, at a value of DM 1.98-2.10/kg. 

The indentation fracture mechanics approach likens abrasive-workpiece interactions for grinding of ceramics to small-scale indentation events. The deformation and fracture patterns observed for normal contact with a Vickers pyramidal indentor under an applied load P are illustrated in Figure 3.1. A zone of plastic deformation is foimd directly imder the indentor. Two principal crack systems emanate from the plastic zone med-ian/radial and lateral cracks. Median/radial cracks are usually associated with strength degradation and lateral cracks with material removal. 

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