Costo de elaboración de las películas biodegradables

Floe strength may be defined as the resistance to fragmentation by shear forces; the ability to withstand these forces depends on the physical and chemical bonds which hold it together. Agitation may assist flocculation by increasing the rate of interparticle collision, but it also promotes the rupture of large aggregates by hydrodynamic forces. There are several mechanisms by which floes or aggregates may be broken down; these include deformation due to pressure gradients, fragmentation, erosion due to hydrodynamic shear, and particle/surface or inter-particle collisions (Bell et a l, 1982). Two extreme systems can be described when characterising the breakup of floes, namely liquid droplets and brittle solids. Liquid droplets in a turbulent or laminar flow field deform as a function of the relative velocity across the droplet (Choulaloyou et a l, 1977). For break-up to occur, the forces of distortion must overcome the surface energy of the droplet. The abrasion of brittle crystals in liquid suspensions describes the other extreme condition. The removal of protruding edges (Nienow & Conti, 1978) proceeds as a first order process with respect to the particle concentration. The frequency of collisions is dependent upon the power being dissipated into the medium. Deformation of floe structure occurs in response to relative velocity ie> variations in velocity over length scales comparable to the aggregate size. Larger eddies tend to carry the floe along, whilst smaller ones have little effect. The mechanical properties of floes will place them somewhere in the range between brittle particle abrasion and liquid droplet break-up. Isoelectric soya precipitate aggregates exposed to a laminar shear rate of 2000 s'^ show an initially rapid decrease in aggregate size followed by a slow steady decrease (Hoare, 1982). Exposure to shear for up to 50 hours does not attain the equilibrium value predicted from orthokinetic aggregation and shear break-up.

There has been shown to be a correlation between floe size and the strength of the aggregate for a given rate of shear (Smith & Kitchener, 1978). Here, the shear break­ up of floes may be accompanied by the scission of bridging polymers and increased

occupancy of available sites, which leads to lower effectiveness of subsequent collisions in forming new floes. Size distributions of protein precipitate aggregates under laminar shear (Hoare, 1982) indicating that aggregate break-up is by fragmentation, since the floe size decreases rapidly during the first period of shear; this proceeds until spherical aggregates are obtained. Further break-up is due to the erosion of primary particles. The fragment size is a weak function of the shear rate (Bell & Dunnill, 1982) whereas the rate of break down, especially of large aggregates, is strongly dependent on the shear rate. From the simplified Smouluchowski equation, the rate of flocculation was taken to be directly proportional to the shear rate (G), implying that high shear rates would reduce the time required to produce aggregates of a given size. But in practice upper limit of shear rates for a given aggregate size will exist, since high shear rates will cause rupture.

Healy and Le M er (1963) suggested that when a suspension flocculated by polymer is exposed to prolonged agitation, the extended polymer segments fold back and adsorb to surface sites, thus producing floes which are less resistant to shear. Further adsorption occurs on the newly exposed particle surface thus leading to redispersion. From their model of bridging flocculation, the maximum aggregation of the suspension occurs at half the surface coverage, whilst the strength of the floes is determined by the efficiency of bridge formation. Birkner and Morgan (1968) suggested that the hydrodynamic forces cause a limited amount of polymer desorption, or polymer rearrangement due to chain cleavage, leading to floe breakage. Aggregates formed by particle destabilisation due to added salts were less susceptible to breakup than those aggregates formed by polymers; however Smith and Kitchener (1978) reported the opposite. As large floes (due to their size) are more susceptible to rupture by hydrodynamic forces and they have proposed that the strength of a floe of a given size will depend on the number of chains adsorbed per unit cross-sectional area.

When floes are disrupted by shear, changes in the configuration of the adsorbed polymer chain may occur, thus reflocculation may or may not occur when the shear is reduced or stopped. Floes produced by a ’charge neutralisation’ mechanism are veiy sensitive to shear, but are readily reformed once the shear is stopped. Floes produced through a bridging mechanism are relatively stronger; when exposed to

excessive shear they show irreversible breakage of the structure. Sikora and Stratton (1981) noted that by varying the charge densities of polyvinylamine, they were able to produce a ’charge neutralisation’ or a ’bridging’ mechanism to flocculate latex particles. It was noted that floes produced using a polymer with a high charge density (charge patch flocculation) readily reformed to the original state on cessation of shearing. When floes produced by a bridging mechanism (low charge density polymer) were subject to shear this led to an irreversible break down of floe structure. It was observed that reconformation of the polymer had occurred as well as polymer scission. Sikora and Stratton (1981) also postulated that when the polymer chain undergoes mid-point scission, its charge would cause it to be reabsorbed onto the particles surface.

Placek and Teague (1988) reported that the flocculation of paper fines was improved by flocculation within various industrial disc-stack centrifuges. Flocculation prior to centrifugation gave poorer results. The high shear accompanying tangential acceleration provided fast and intensive mixing; the floes formed under these conditions were more shear resistant and gave improved settling characteristics. They also reported that the broth flocculated in the bowl gave a more compacted sludge compared to pre-flocculated feed material.

Theoretical and empirical models for the prediction of shear breakup of floes have not proved to be entirely successful due to the time dependent properties of the precipitate during breakup. The simplest correlations for breakup usually relate the maximum stable size of an aggregate to the shear rate. Tomi and Bagster (1978) showed that the maximum stable size of floes is related to 0.5 of the power of the average shear rate (G); Smith and Kitchener (1973) reported that the maximum diameter is proportional to the -0.2 power of the average shear rate.