Polymer melt filtration

As demands have grown for finer degrees of filtration in available filter media, the extruded fibres used in these media must also be made to a smaller and smaller diameter. This means that the extrusion process must produce smaller diameter material, and so the nozzles must be finer, which makes them more easily blocked by contaminant particles - unless these, in turn, are filtered from the melt. The filter media used for this task of polymer melt filtration have been discs of sintered powder, fibre or mesh. 

For some critical applications (such as polymer melt, beverage, or pharmaceutical filtration), it may be important to avoid cartridges that have a nap or fuzz on the fiber used, because these extremely fine fibers tend to break off and drift through the cartridge and go out with the finished product, thereby creating a isual acceptance problem, if not outi ight contamination. 

The Vinyloop process is based on the selective dissolution of PVC used in composites applications like cable insulation, flooring, tarpaulins, blisters, etc. After removal of insoluble parts like metals, rubber or other polymers, the PVC is reprecipitated with all additives by introduction of a non-solvent component whieh will form with the seleetive solvent an azeotropie mixture. By using typical conditions, the process is able to reeover a pure PVC eompound powder ready for use without any additional treatment like melt filtration or a new pelletisation (speeific characteristics of the powder are average diameter of 400 microns and bulk density above 600 kg/ eub.m). All the solvents used are eompletely reeyeled and reused. PVC compounds recovered in the Vinyloop process can be reused in a closed-loop recycling scheme... 

Pahl, M.H., Fundamental Principles of the Filtration of Non-Newtonian Fluids, Chapter 1 in Filtration of Polymer Melts, English translation by WelUng, M. S., VDl-Verlag GmbH, Dusseldorf (i98t)... 

Melt filtration systems are commonly employed in pigment master-batch production and in situations where the presence of defects in the compound may have a critical effect on its subsequent processing or properties. This is vitally important, for example, in fibre-spinning operations involving extrusion of polyester or polyamide through fine spinneret plates [162], and in minimizing breakdown of polymer cable insulation subjected to electrical stress [163]. 

An important application field for stainless steel fibers is the textile sector, in which 0., i to 6% of these fibers are incorporated to endow carpets, protective clothing etc. with an antistatic finish. A further application is protection against electromagnetic pulses, interference and charging. Tungsten fibers with a diameter of 12 pm are used for boron or SiC deposition and as light bulb filaments. Furthermore, metal fibers are used in the filtration of polymer melts and corrosive liquids, as well as for electrodes with high surface areas. 

Glycolysis is the simplest and oldest method of PET depolymerization. The first patents on PET glycolysis were filed more than 30 years ago.3-9 The method involves the reaction of PET, under pressure and at temperatures in the range 180-240 °C, with an excess of glycol, usually ethylene glycol, which promotes the formation of BHET.10-13 This monomer has to be purified, normally by melt filtration under pressure, prior to its use in the production of new PET polymer. Colours present in the starting PET wastes are not usually removed by glycolysis. The depolymerization is carried out in the presence of a transesterification catalyst, usually zinc or lithium acetate. 

The spin pump (metering pump) delivers the molten polymer to a spin pack, which consists of a top cap and a breaker plate to distribute polymer evenly, then into a filtration media and out of the spinneret. The filtration media contains either different layers of special sand, layers of different size of stainless steel screens, or sintered metals. In addition to removing the foreign particles, gel particles and undesirable conglomerate additives, filtration may also improve the polymer melt homogeneity due to its torturous path and high shear of the filtration media. 

The filtration of polymer melt is normally supplied immediately downstream from the extruder. This filtration can range from a coarse single screen supported by a breaker plate to a very fine nonwoven metal filter contained in a separate housing that allows the filter to be changed without interruption of the process. The choice of filtration depends on the process, the feed material, and the product. 

An important element in thermoplastics recycling is melt filtration to separate out unmelted or unmeltable shares in the polymer. These particles are held back by metal textiles or perforated plates and flushed out of the poymer melt with a partial current. The loss of polymer melt mass amounts to about 3-5 % plus the mass of the separated particles. 

Mechanisms and design models for filtration other than that forming a filter cake are also described, and so is non-Newtonian filtration. It is well known that a filtration cycle displays characteristics of these other filtration modes prior to the occurreace of cake filtration. This period is usually very short even at low concentrations of slurry. The other filtration modes are particularly relevant to clarifying filtration, at low to very low concentrations (less than 1% solids by volume). This type of filtration is inq>ortant industrially, and is typical of the deep-bed filtration discussed in Chapter 6. Filtration of non-Newtonian liquids is somewhat more specialised, but can be found in the filtration of oils and polymer melts. 

Filtration of Polymer Melts, VDI PuhUcation, Duesseldorf, Germany (1981)... 

Machinery used to prepare particulate-filled polymer composites may be classified in terms of the applied shear intensity and whether the operation is of a batch or continuous nature. Furthermore, most compounding processes are critically dependent on the efficiency of ancillary equipment to undertake various functions, including additive and polymer feeding, melt filtration and pelletising procedures. 

In cases where elimination of agglomerates is essential to ensure that optimum compound properties are achieved, melt filtration can be used. This is important in pigment masterbatch production or in coloured compounds to be spun into fibres, where agglomerates can lead to fibre breakage. Removal of impurities or large defect particles by melt filtration can also increase the fatigue life of FIDPE pipe and the resistance to breakdown of polymer subjected to an electrical stress [71, 72]. 

Melt filtration is typically used to remove contaminants from polymer melts dining the extrusion process. There is a mechanical separation of the... 

Melt filtration systems can very effectively remove particulate contaminants from polymer melts, for example, continuous systems provided by Process Developments [10] and by Gneuss. Kauferle (Remaker) produced a prototype device capable of causing separation of the melt from high concentrations of aluminium from polystyrene in laminates, or copper from polyethylene in telephone wire, without the need for disposable filter screens. The solid and particle contaminant-free thermoplastic exited from one port on the separation head, while a very high concentration of contaminant embedded in... 

Filtration can be applied in all stages of the process in a central filter in the polymer melt line, or at the end of the line, in the spinning assembly. Central filters are common in large-scale melt-spinning processes, for example, staple fiber production. Two parallel filters may be used to enable filter deaning without interruption... 

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