Residue catalysts

LLDPE by itself does not present any health-related hazard on account of its chemical inertness and low toxicity. Consequently, film, containers, and container Hds made from LLDPE are used on a large scale in food and dmg packaging. Some LLDPE grades produced with unsupported metallocene catalysts have an especially high purity due to high catalyst productivity and a low contamination level of resins with catalyst residue. FDA approved the use of film manufactured from these resins for food contact and for various medical appHcations (80). However, if LLDPE articles contain fillers, processing aids, or colorants, thek health factors must then be judged separately. 

TiCl catalysts produced by the reduction of TiCl with Al(C2H 2d> subsequentiy treated first with an electron donor (diisoamyl ether), then with TiCl, are highly stereospecific and four to five times more active than d-TiCl (6). These catalysts were a significant advance over the earlier TiCl systems, because removal of atactic polymer was no longer required. They are often referred to as second-generation catalysts. The life of many older slurry process faciUties has been extended by using these catalysts to produce "clean" polymers with very low catalyst residues. 

Polymerization in Hquid monomer was pioneered by RexaH Dmg and Chemical and Phillips Petroleum (United States). In the RexaH process, Hquid propylene is polymerized in a stirred reactor to form a polymer slurry. This suspension is transferred to a cyclone to separate the polymer from gaseous monomer under atmospheric pressure. The gaseous monomer is then compressed, condensed, and recycled to the polymerizer (123). In the Phillips process, polymerization occurs in loop reactors, increasing the ratio of available heat-transfer surface to reactor volume (124). In both of these processes, high catalyst residues necessitate post-reactor treatment of the polymer. 

Gas-phase polymerization of propylene was pioneered by BASF, who developed the Novolen process which uses stirred-bed reactors (Fig. 8) (125). Unreacted monomer is condensed and recycled to the polymerizer, providing additional removal of the heat of reaction. As in the early Hquid-phase systems, post-reactor treatment of the polymer is required to remove catalyst residues (126). The high content of atactic polymer in the final product limits its usefiilness in many markets. 

In the 1970s, Solvay iatroduced an advanced TiCl catalyst with high activity and stereoregulahty (6). When this catalyst was utilized ia Hquid monomer processes, the level of atactic polymer was sufftciendy low so that its removal from the product was not required. Catalyst residues were also reduced so that simplified systems for post-reactor treatment were acceptable. Sumitomo has developed a Hquid monomer process, used by Exxon (United States), ia which polymer slurry is washed ia a countercurrent column with fresh monomer and alcohol to provide highly purified polymer (128). 

Montedison and Mitsui Petrochemical iatroduced MgCl2-supported high yield catalysts ia 1975 (7). These third-generation catalyst systems reduced the level of corrosive catalyst residues to the extent that neutralization or removal from the polymer was not required. Stereospecificity, however, was iasufficient to eliminate the requirement for removal of the atactic polymer fraction. These catalysts are used ia the Montedison high yield slurry process (Fig. 9), which demonstrates the process simplification achieved when the sections for polymer de-ashing and separation and purification of the hydrocarbon diluent and alcohol are eliminated (121). These catalysts have also been used ia retrofitted RexaH (El Paso) Hquid monomer processes, eliminating the de-ashing sections of the plant (Fig. 10) (129). 

Polymers and higher a-olefins are not toxic their main potential health hazards are associated with residual monomer, antioxidants, and catalyst residues. 

After epoxidation a distillation is performed to remove the propylene, propylene oxide, and a portion of the TBHP and TBA overhead. The bottoms of the distillation contains TBA, TBHP, some impurities such as formic and acetic acid, and the catalyst residue. Concentration of this catalyst residue for recycle or disposal is accompHshed by evaporation of the majority of the TBA and other organics (141,143,144), addition of various compounds to yield a metal precipitate that is filtered from the organics (145—148), or Hquid extraction with water (149). Low (<500 ppm) levels of soluble catalyst can be removed by adsorption on soHd magnesium siUcate (150). The recovered catalyst can be treated for recycle to the epoxidation reaction (151). 

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). 

Erom 1955—1975, the Ziegler-Natta catalyst (91), which is titanium trichloride used in combination with diethylaluminum chloride, was the catalyst system for propylene polymerization. However, its low activity, which is less than 1000 g polymer/g catalyst in most cases, and low selectivity (ca 90% to isotactic polymer) required polypropylene manufacturers to purify the reactor product by washing out spent catalyst residues and removing unwanted atactic polymer by solvent extraction. These operations added significantly to the cost of pre-1980 polypropylene. 

Catalyst residues, particularly vanadium and aluminum, have to be removed as soluble salts in a water-washing and decanting operation. Vanadium residues in the finished product are kept to a few ppm. If oil-extended EPDM is the product, a metered flow of oil is added at this point. In addition, antioxidant, typically of the hindered phenol type, is added at this point. 

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. 

Factors affecting laboratory polymerisation of the monomer have been discussed" and these indicate that a Ziegler-Natta catalyst system of violet TiCl3 and diethyl aluminium chloride should be used to react the monomer in a hydrocarbon diluent at atmospheric pressure and at 30-60°C. One of the aims is to get a relatively coarse slurry from which may be washed foreign material such as catalyst residues, using for example methyl alcohol. For commercial materials these washed polymers are then dried and compounded with an antioxidant and if required other additives such as pigments. 

The reaction takes place at low temperature (40-60 °C), without any solvent, in two (or more, up to four) well-mixed reactors in series. The pressure is sufficient to maintain the reactants in the liquid phase (no gas phase). Mixing and heat removal are ensured by an external circulation loop. The two components of the catalytic system are injected separately into this reaction loop with precise flow control. The residence time could be between 5 and 10 hours. At the output of the reaction section, the effluent containing the catalyst is chemically neutralized and the catalyst residue is separated from the products by aqueous washing. The catalyst components are not recycled. Unconverted olefin and inert hydrocarbons are separated from the octenes by distillation columns. The catalytic system is sensitive to impurities that can coordinate strongly to the nickel metal center or can react with the alkylaluminium derivative (polyunsaturated hydrocarbons and polar compounds such as water). 

Electron beam-initiated modification of polymers is a relatively new technique with certain advantages over conventional processes. Absence of catalyst residue, complete control of the temperature, a solvent-free system, and a source of an enormous amount of radicals and ions are some of the reasons why this technique has gained commercial importance in recent years. The modification of polyethylene (PE) for heat-shrinkable products using this technique has been recently reported [30,31]. Such modification is expected to alter the surface properties of PE and lead to improved adhesion and dyeability. 

Cut Out Separations. This can produce significant savings regardless of the stage of the process to which it is applied, provided efficiency of other parts of the process can be maintained. Avoidance of treatments to feedstocks and intermediates is clearly advantageous as is the removal of the need to clean up catalyst residues in high density polyethylene. 

Residual catalyst (expressed as ash content). The actual residues depend on the manufacturing process used and on the characteristics of the polymer. The so-called new generation HD-PE processes such as the Solvay process use superactive catalysts which produce polymers with a low ash content and, hence, low or negligible odor. Some narrow MWD (Molecular Weight Distribution) resins also have lower catalyst residues than their wide MWD counterparts. 

Antioxidant additive in the polyethylene resin. While such an additive can prevent oxidation, and thus odor, it also can contribute directly to the odor. If an antioxidant is needed, it must be FRA approved, should have a high melting point, and should be used at a minimum level consistent with the extrusion process. Catalyst residues and antioxidants present in polyethylene sometimes interact to form odorous products. 

In contrast to bulk polymerization, solution polymerization provided soluble polymers with high molecular weights using low FeCl3 concentration at 120-140 C.31 A major disadvantage of the above approaches is that all the metal-halide catalysts need to be removed, since the catalyst residue will deteriorate die thermal stability and electrical and other properties. 

Because of the absence of chain limiter, the catalyst itself may initially act as the chain limiter (Fig. 8.22). The catalyst reacts with the olefinic regions of the polymer backbone and causes chain scission to occur, forming two new chains. The reactive carbene which is produced then moves from chain to chain, forming two new chains with each scission until the most probable molecular weight distribution is reached (Mw/Mn = 2), producing linear chains end capped with [Ru] catalyst residues. 

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