Residues of Catalysts

The polymer may also contain residues of catalyst left over from the original polymerisation process. These may provide possible initiation sites for degradation processes. 

The color of the final product primarily depends on the qualification of the raw materials, TPA, DMT and EG. The content of heavy metals in TPA, residues of catalysts employed during oxidation of p-xylene, and polymer processing affect the final color of the polymer. The tendency of certain catalysts, such as titanium or tin derivatives, to make the polyester yellowish in color is well established. The conversion during esterification is prolonged due to larger TPA particles or their hardness. Color can be influenced by these factors, as well as by chemical impurities in the raw materials, such as water, aldehydes or the quality of insufficiently recovered EG. Similar effects on color can be observed as a result of impurities caused by additives, particularly from less purified Sb2C>3. The quality of the latter can be assessed simply by the color of its solution in EG. 

There is interest in trace metal levels in both natural and synthetic fibres and fabrics but perhaps most interest is in synthetic fibres as these may contain residues of catalysts, treatments or stabilising agents. Reviews have been published of trace-metal analysis of rayon, polyamide, polyester and polypropylene fibres [178] and of cotton fabrics, especially for flame... 

Fig. 5. Comparison of supernatant liquids and residues. Catalyst 2 mg. Pd in 40 cc. of 50% alcohol. Acceptor 0.5 oc. CaHsNOj. A, supernatant liquid of sample centrifuged at 500 r.p.s. B, supernatant liquid of sample centrifuged at 200 r.p.s. C, residue of catalyst centrifuged at 200 r.p.s. D, residue of catalyst centrifuged at 500 r.p.s. E, normal Hz-reduced PVA-Pd. All measurements are converted to 25°C. and 760 mm.

It should not be forgotten that, in addition to additives which are deliberately introduced into the resin, plastics may contain substances that are residues of substances produced or introduced at some stage of polymerization or processing, such as residues of catalysts, solvents, or unreacted monomer or substances which have migrated into the polymer from the contents of the package or from its surroundings. 

Predicting which bonds, if any, are iikeiy to be broken on heating is not a simpie process. Not aii sites in a poiymer are equaiiy vuinerabie to thermoiysis. The weakest bonds in the poiymer are those most iikeiy to break. Bonds in impurities which have become incorporated into poiymer molecules, including traces of monomers and residues of catalysts from synthesis, are more vulnerable than the same bonds in the polymer s backbone. Terminal groups and O—O bonds formed as a product of oxidation are also more vulnerable than the same bonds located in the backbone. 

Therefore, the radical yield (f) is 0 monomer units of one macromolecule or belong to different macromolecules. At the origin of the chain oxidation may participate impurities of transition metals, residues of catalysts or initiators, etc. These impurities get into the polymer as a result of receiving or processing the polymer. Table. 1... 

Polymer is degraded by heat, energy, UV or residues of catalyst and generates alkyl radicals. This alkyl radical reacts with oxygen and form peroxy radicals. These peroxy radicals abstract hydrogen from other polymer and forms alkyl radicals and hydroperoxide. The decomposition of hydroperoxide to alkoxy and hydroxyl radicals induces additional decomposition of the polymer chain. In order to stop the radical chain reaction of degradation, stabilisers such as phenolic antioxidant, phosphites, thioether and hindered amine light stabilisers (HALS) are added. 

The low molecular weight material segregated at the surface may be more or less disordered than in the ideal single crystal. In the polymer melt there will also exist polymer that has become oxidized, residues of catalysts and processing aids. Often such materials will segregate to an interface. It is therefore not... 

Non-rubber constituents present in synthetic polyisoprenes are limited to small residues of catalyst and added stabiliser (BHT or similar phenolic antioxidant in non-staining types and DP PD + PBNA in SKI 3 and Carom 2230) and both ash and acetone extract contents are low, totalling 1-15%. Natural rubbers generally contain non-rubbers, consisting of proteins, fatty acids and phospholipids, which total 3-6% depending... 

Arsenic Peroxides. Arsenic peroxides have not been isolated however, elemental arsenic, and a great variety of arsenic compounds, have been found to be effective catalysts ia the epoxidation of olefins by aqueous hydrogen peroxide. Transient peroxoarsenic compounds are beheved to be iavolved ia these systems. Compounds that act as effective epoxidation catalysts iaclude arsenic trioxide, arsenic pentoxide, arsenious acid, arsenic acid, arsenic trichloride, arsenic oxychloride, triphenyl arsiae, phenylarsonic acid, and the arsenates of sodium, ammonium, and bismuth (56). To avoid having to dispose of the toxic residues of these reactions, the arsenic can be immobi1i2ed on a polystyrene resia (57). 

The use of alkali or alkaline-earth sulfides cataly2es the reaction so that it is complete in a few hours at 150—160°C use of aluminum chloride as the catalyst gives a comparable reaction rate at 115°C. When an excess of sulfur is used, the product can be distilled out of the reactor, and the residue of sulfur forms part of the charge in the following batch reaction. The reaction is carried out in a stainless steel autoclave, and the yield is better than 98% based on either reactant. Phosphoms sulfochloride is used primarily in the manufacture of insecticides (53—55), such as Parathion. 

Phosgene addition is continued until all the phenoHc groups are converted to carbonate functionahties. Some hydrolysis of phosgene to sodium carbonate occurs incidentally. When the reaction is complete, the methylene chloride solution of polymer is washed first with acid to remove residual base and amine, then with water. To complete the process, the aqueous sodium chloride stream can be reclaimed in a chlor-alkah plant, ultimately regenerating phosgene. Many variations of this polycarbonate process have been patented, including use of many different types of catalysts, continuous or semicontinuous processes, methods which rely on formation of bischloroformate oligomers followed by polycondensation, etc. 

Acidic contaminants are poisonous to the alcoholysis catalysts and must be avoided. If the oil has a high acid number, or there are high acidity residues left in the reactor from the previous batch, such as sublimed phthaUc anhydride condensed under the dome of the reactor, the reaction can be severely retarded. A longer batch time or additional amount of catalyst is then required. Both are undesirable. 

Catalyst Development. Traditional slurry polypropylene homopolymer processes suffered from formation of excessive amounts of low grade amorphous polymer and catalyst residues. Introduction of catalysts with up to 30-fold higher activity together with better temperature control have almost eliminated these problems (7). Although low reactor volume and available heat-transfer surfaces ultimately limit further productivity increases, these limitations are less restrictive with the introduction of more finely suspended metallocene catalysts and the emergence of industrial gas-phase fluid-bed polymerization processes. 

The presence of catalyst residues, such as alkali hydroxide or alkali acetate, a by-product of the hydrolysis reaction, is known to decrease the thermal stability of poly(vinyl alcohol). Transforming these compounds into mote inert compounds and removal through washing are both methods that have been pursued. The use of mineral acids such as sulfuric acid (258), phosphoric acid (259), and OfXv o-phosphotic acid (260) has been reported as means for achieving increased thermal stability of the resulting poly(vinyl alcohol). 

The electrical properties of polypropylene are very similar to those of high-density polyethylenes. In particular the power factor is critically dependent on the amount of catalyst residues in the polymer. Some typical properties are given in Table 11.3 but it should be noted that these properties are dependent on the antioxidant system employed as well as on the catalyst residues. 

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