Polymer Choice

In decanter applications, the higher molecular weight polymers predominate. Nevertheless, occasionally some lower molecular weight polymers are found very effective when used on some decanter centrifuges. Cross-linked polymers are finding frequent use on decanters, with both municipal and, particularly, industrial effluents. 

When a sample of sludge arrives in the laboratory for flocculant assessment, there are a number of common tests which are conducted. Its settling rate under gravity is observed and its pH is measured. A small sample, about 50 ml, is spun in a laboratory test-tube centrifuge for a fixed time, say 5 to 10 minutes. The g-level at the tip of the centrifuge will be of the order of 1500 to 2000 g. The volume of sediment will be recorded. 

The analyses mentioned so far all help to form a picture of the sludge to compare with past experience. This experience will enable the choice of a range of likely polymers that might be effective. A selection of anything from 10 to 20 polymers would be made, and preferably fresh samples of the flocculant will be made. Alternatively, already prepared concentrated solutions will be diluted to provide, say, 100 ml samples at 0.1% concentration. Alternatively, for expediency and economy, old (but not too old) samples will be used initially, and then a narrower range of fresh polymer samples will be made up on the basis of the initial assessment. The initial assessment is made by adding aliquots of each polymer to separate measured quantities of sludge. 

There are many ways of performing this initial polymer evaluation, both empirical and rigorous [4], each depending upon the preferences of the analyst, but many are similar with the same objective. One example follows. 

From the initial assessment, a selection of, say, three to six polymers will be made. Any polymer which does not perform on the bench will not perform in the decanter. Any polymer which does perform on the bench is a candidate for the centrifuge, but it is not a guarantee that it will work well there. 

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Weight saving is one of the reasons leading to polymer choice. 

Hence, there are two criteria to properly design a container for water transport. The first is to insure the polymer is rate limiting (Equation 7), and the second is to insure a minimum weight loss over the shelf life. To properly apply these equations, the wall thickness required for a given polymer is found from Equation 8. This wall thickness then is the minimum value to use in Equation 7. That is, apply Equation 7 with the P/l from Equation 8. If the L from Equation 8 is less than 4, then increase the wall thickness with the criteria that Equation 7 must be greater than or equal to 4. If these conditions cannot be met practically or economically, the designer will need to iterate on polymer choice to insure these requirements can be met. 

Thermoplastic powders are nfixed with water to form slurry that is coated onto reinforcing fibers or webs. When the water is evaporated, the powder sticks to the reinforcement. These prepregs are drapable and pliant. However, they are expensive and limited in polymer choice. 

PLGA (Cutright and Hunsuck, 1971 Athanasiou et al., 1996), which was employed as suture by taking advantage of its biodegradation ability. So far, the medical applications have not changed drastically and the main polymer choices are still the classical types of copolymers based on polylactic acid (PLA) and polyglycolic acid (PGA) as shown in Table 7.6. 

Yamashita et al. (2003) investigated the dissolution in acidic medium of solid dispersions containing the macrolide lactone tacrolimus in an amorphous state comparing three different polymers (HPMC, PVP, and PEG 6000) as the carrier. Rapid dissolution and supersaturated concentrations of tacrolimus up to 25-fold higher than the equilibrium solubility (2 mg/mL) were observed. Even though the polymer choice did not affect the maximum degree of supersaturation, it was only HPMC that could fully inhibit precipitation for up to 24 h. 

For general purpose rubbers, the quantity and type of monomer incorporated for cure may be of vital importance. This will determine the method and rate of cure that is possible. Several of the more exotic polymers have recently become available with termonomers incorporated to allow crosslinking by peroxides rather than by the use of more exotic chemicals. This has provided a greater degree of latitude to the chemist in polymer choice. 

Polymers used to insulate plugs and sockets include PA and TPE. However, for plugs moulded on to cables the universal polymer choice for plug bodies has been PVC which can tolerate temperatures up to 60 "C. 

In addition to these developments, work has been carried out to improve the standard range of nitrile rubbers by designing polymers with better colour and reduced mould-fouling characteristics, and an attempt to rationalize the range and thereby simplify polymer choice has also occurred. 

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