LDPE production

The LDPE production with tubular reactors (see Section 5.1) requires some sophisticated control valves [45]. The let-down valve (Fig. 4.2-6 B) controls the polymerization reaction via the pressure and temperature by a high-speed hydraulic actuator (9) together with an electronic hydraulic transducer. The position of the valve relative to the stem is determined by a high-resolution electronic positioner (7). The cone-shaped end of the valve stem (2), as well as the shrunk valve seat (3) are made from wear-resistant materials (e.g., sintered tungsten carbide) in order to tolerate the high differential pressure of around 3000 bar during the expansion of the polymer at that location. 

With a single passage, approximately 20 - 40% of the ethylene introduced is converted into polyethylene in the reactor. On the basis of current reactor dimensions, this corresponds to a LDPE production of 100,000 - 300,000 t/a per reactor. 

No comonomer required (homopolymer) for LDPE production Up to 10%-15% comonomer required (in case of copolymer)... 

A topical powder product may be marketed in a sifter-top container made of flexible plastic tubes or as part of a sterile dressing (e.g., antibacterial product). The topical formulations in a collapsible tube can be constructed from low-density polyethylene (LDPE), with or without a laminated material. Normally, there is no product contact with the cap during storage. Thus usually there is no cap liner, especially in collapsible polypropylene screw caps. Normally separate applicator devices are made from LDPE. Product contact is possible if the applicator is part of the closure, and therefore an applicator s compatibility with the drug product should be established, as appropriate (e.g., vaginal applicators). 

ExxonMobil Chemical Co. Polyethylene, high pressure LDPE Ethylene and EVA State-of-the-art reactor provides broadest scope for LDPE products including high-clarity films to medium density polymers 4 1999... 

Chain scission. The midchain radical structure formed by intra- or intermolecular transfer to polymer is less reactive than a chain-end radical. Under higher temperature conditions, the radical may undergo -ffagmentation (chain scission) as shown in Scheme 3.10 for BA. As well as lowering polymer MW, sdssion produces an unsaturated chain end that can react further (Scheme 3.7b). Scission is important for acrylate polymerizations at temperatures > 140°C [18,21], is a dominant mechanism in styrene polymerizations at 260-340°C [15], and also occurs during LDPE production [14]. Kinetic treatment is difficult, as scission is coupled with LCB and/or SCB formation. 

High-pressure autoclaves for LDPE production represent a special class of (usually elongated) reactors, the analysis of which is handled via combinations of well-mixed and plug flow (with recycle) reactor elements. Information and models can be found in references 122-126. 

LDPE density was controlled by comonomer content, while MWD by H2 concentration. H2O improved conversion up to the saturation concentration. Retrofitting the ICI system reduced the LDPE production cost by 15-20 %... 

LDPE Copolymers. A variety of comonomers can be added to the polymerization of ethylene to make copolymers. The free-radical polymerization mechanism of LDPE production allows for the copolymerization of polar comonomers. At this time, the incorporation of polar comonomers is unique to LDPE. The transition metals used to catalyze HDPE and LLDPE production are generally poisoned by polar comonomers and therefore, only copolymers containing alpha-olefins like 1-butene, 1-hexene, and 1-octene can be made. Because the polar copolymers can be made only by the LDPE process, they command a premium in the market. The most common comonomers (and their corresponding copol5uners) are vinyl acetate (EVA), methyl acrylate (EMA), ethyl acrylate (EEA), and acrylic acid... 

A majority of the LDPE used in North America is shipped in 82-ton rail cars. Other common forms of shipment are 18-ton bulk trucks, 500-kg boxes, and 25-kg bags. In general, no special safety labels are needed for LDPE products. In other parts of the world, the most common mode of shipment is in 25-kg bags. 

Figure 3.7 Flow diagram showing LDPE production...

Off-gases from the extruding section in LDPE production are rich in monomers. By sucking off the fumes from the extrusion section, the emission of monomers is reduced. The off-gases are collected and treated in a thermal oxidising unit. 

LDPE production takes place in a single-phase ethylene/PE mixture that allows the reaction to proceed as a free-radical-initiated solution polymerization. This particular reaction mode leads inevitably to a high-pressure process. Pressures of 2000 bar and temperatures above 160 °C are required to dissolve the already formed PE in the unreacted ethylene. 

LDPE production is a radical high-pressure process. Despite the high investment and higher operation cost related to this process, LDPE processes are still in use as the special branching pattern of the product cannot be achieved with low-pressure techniques. Some applications, for example, PE extrusion, require the properties of LDPE. 

Scission events can occur in any system where mid-chain radicals are formed. However, scission is more temperature-activated than H-abstraction and thus becomes important only at elevated temperatures. The reaction is not believed to occur during butyl acrylate polymerization at 75 C [37], but is shown to be important at 140°C [29, 45], Scission is a dominant mechanism in styrene polymerizations at 260-340°C [26], and also occurs during LDPE production [30]. Scission of midchain radicals formed via intermolecular transfer to polymer can have a significant effect on the breadth and the shape of polymer MWD [46]. 

Additives typically used to modify the properties of extruded and moulded LDPE products can be used to advantage with E/CO. Due to its polar nature, E/CO appears to have greater affinity for certain additives, such as fatty acid amide slip promoters. In some cases, slightly higher concentrations of additives may be needed to achieve the desired surface properties, especially in the higher CO copolymers. 

LDPE, ethylene copolymers molding and extrusion compounds LDPE, ethylene copolymer molding and extrusion compounds LDPE products for plastic processing... 

Applications include film for packaging manufactured by the tube-blowing process, which consumes some 70% of LDPE production a miscellany of uses takes the remaining 30%, including ... 

Two different high-pressure processes using autoclave or tubular reactors are applied for LDPE production. The autoclave process was developed by ICI, whereas the tubular reactor process was developed by BASF Aktiengesellschaft (predecessor of LyondellBasell s Lupotech T process). Monomer conversion rates of the adiabatic autoclave process can reach 25% compared to values up to 40% for tubular reactors, where the heat of polymerization can be partly removed through the jacketed reactor tubes via circulating cooling water. 

With the invention of linear low-density polyethylene (LLDPE) by coordinative catalysis using ethylene and alpha-olefins as comonomers in the 1970s, many consultants predicted a significant replacement of the entire LDPE product portfolio by the novel LLDPE resins. As a consequence, only limited activities for further development of the LDPE processes have been undertaken by the market players. 

Compared to other polymer production processes and even among polyolefin processes, the high-pressure LDPE production technology is highly suitable for... 

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