MELT FILTRATION

Foreign contamination is typically first discovered by quality control checks of the finished product or by the loss of the web for film processes. If a melt filtration system is installed downstream of the extruder, the larger size particles will be collected. After the contaminants are collected they must be analyzed for composition. Some types of contaminants are easily identified using a microscope or hand lens and include paper and cloth fibers, dirt, and metal fragments. Other contaminants such as gels or foreign resins are not as easily identified, and their identification often requires advanced analytical procedures. Many resin manufacturers offer these types of services to their customers. After the contaminant is identified, the source must be determined and then eliminated. Elimination of the source can be simple for common contaminants but can be a challenge for contaminants that exist at a very low level. 

Mesh Size (M), wires/inch Wire Diameter (d ), mm Average Opening (d ), mm 

The pressure change required to operate a screen changer system with fresh screens can he estimated using a method developed by Carley and Smith [3] and later modified by Todd [4] for a power law fluid. 

Mesh Size Warp Wire Diameter p), mm Weft Wire Diameter [dj, mm Average Opening [do), mm 

The pressure change across a breaker plate can also limit a process. The pressure change across the plate can be estimated by using simple equations for flow through a tube [5]. The calculation ignores end effects and is shown in Eq. 11.6  

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Another feature of an extruder is the presence of a gauze filter after the screw and before the die. This effectively filters out any inhomogeneous material which might otherwise clog the die. These screen packs as they are called, will normally filter the melt to 120-150 fim. However, there is conclusive evidence to show that even smaller particles than this can initiate cracks in plastics extrudates e.g. polyethylene pressure pipes. In such cases it has been found that fine melt filtration ( 45 p.m) can significantly improve the performance of the extrudate. 

Methanolysis products are separated and purified by distillation. BHET, the monomer obtained by PET glycolysis, is normally purified by melt filtration under pressure. One of the problems encountered in neutral hydrolysis of PET is that the terephthalic acid isolated contains most of the impurities initially present in the PET waste. Hence very elaborate purification processes are required to obtain terephthalic acid of commercial purity. 

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... 

Why is melt filtration necessary and how is it accomplished What would an operator observe if a melt filter were to rupture ... 

In this process, a vacuum extrusion step (melt treatment) is followed by a continuous SSP (solid treatment). The process starts with PET flakes which are introduced without pre-drying into a ring extruder. The flakes are dried, melted and degassed inside the extruder. A gear pump builds the necessary pressure for melt filtration, where solid particles are removed. After granulation the pellets are fed to a continuous three-step SSP unit [10]. 

Determination of Hot Melt — Spectrophotometer. Compare the spectral absorbance at 20°C of the ethylene chloride filtrate obtd in 4.4.3.2.1 or 4.4.3.4.1 with that of a standard soln (0.4g of TNT/50ml of ethylene chloride) at 430 millimicrons. The weight of hot melt is detd by reference to a graph prepd in advance from standard hot melt solns. The prepn of the solns and graph are described in 4.4.3.5.14. The TNT + wax + hot melt filtrate collected in4.4.3.3.1, diluted to 50ml with RDX satd benzene may be substituted for the ethylene chloride-TNT soln obtd in 4.4.3.4.1. All graphs and standard solns needed will substitute RDX satd benzene for ethylene chloride... 

B = Wt of evaporating dish plus cover V = Volume of hot melt filtrate pipetted into evaporating dish... 

Into each of six 50-ml volumetric flasks, pipette accurately 25ml of the TNT stock soln, add to each of the flasks respectively, by pipetting accurately, 0.5ml, 1.0ml, 1.5m., 2.0ml, 2.5ml, 3.0ml of the hot melt filtrate. Dilute with ethylene chloride to the mark... 

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]. 

Model investigations undertaken using high-density polyethylene pipe dosed with particulate aluminum flaws of known size showed that resistance to stress rupture increased significantly after removal of larger stress raising particles by melt filtration [164]. 

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. 

Extrusion with melt filtration can then be used as the ultimate way of purifying the material. Since each extrusion step exposes the material to thermomechanical cycles likely to further degrade the material, the number of extrusion steps should be kept to a minimum. For the final extrusion compounding, additives can be used to maintain or improve selected material properties, depending upon the composition and degree of degradation of the material. 

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. 

Because recycled plastics have had a first use and have been environmentally exposed, required conversion processes are often more stringent. For example, compounding may have to be done on a twin rather than a single-screw extruder for dispersive mixing. In addition, compounding purification steps such as devolatilization and melt filtration may be required for some recycled feedstock. Consequently, the compounding process can have a significant impact on the overall conversion cost. Another contributor to conversion cost is the compatibilizer additive. The most effective of these additives cost about 2.00 per pound. This adds 10 cents per pound of product at a 5% level of compatibilizer additive. 

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