Polyethylene point

Ethylene. Under the influence of pressure and a catalyst, ethylene yields a white, tough but flexible waxy sohd, known as Polythene. Polyethylene possesses excellent electric insulation properties and high water resistance it has a low specific gravity and a low softening point (about 110°). The chemical inertness oi Polythene has found application in the manufacture of many items of apparatus for the laboratory. It is a useful lubricant for ground glass connexions, particularly at relatively high temperatures. 

ETHYLENE We discussed ethylene production in an earlier boxed essay (Section 5 1) where it was pointed out that the output of the U S petrochemi cal industry exceeds 5 x 10 ° Ib/year Approximately 90% of this material is used for the preparation of four compounds (polyethylene ethylene oxide vinyl chloride and styrene) with polymerization to poly ethylene accounting for half the total Both vinyl chloride and styrene are polymerized to give poly(vinyl chloride) and polystyrene respectively (see Table 6 5) Ethylene oxide is a starting material for the preparation of ethylene glycol for use as an an tifreeze in automobile radiators and in the produc tion of polyester fibers (see the boxed essay Condensation Polymers Polyamides and Polyesters in Chapter 20)... 

One of the mam uses of the linear a olefins prepared by oligomerization of ethylene is in the preparation of linear low density polyethylene Linear low density polyethylene is a copoly mer produced when ethylene is polymerized in the presence of a linear a olefin such as 1 decene [H2C=CH(CH2)7CH3] 1 Decene replaces ethylene at random points in the growing polymer chain Can you deduce how the structure of linear low density polyethylene differs from a linear chain of CH2 units ... 

Dextrin Polyethylene glycol 400 Use 5 mL of 2% aqueous solution of chloride-free dextrin per 25 mL of 0. IM halide solution. Prepare a 50% (v/v) aqueous solution of the surfactant. Use 5 drops per 100 mL end-point volume. 

Figure 2.5 Shearing force per unit area versus shear rate. The experimental points are measured for polyethylene, and the labeled lines are drawn according to the relationship indicated. (Data from J. M. McKelvey, Polymer Processing, Wiley, New York, 1962.)...

Many initiators attack steels of the AISI 4300 series and the barrels of the intensifiers, which are usually of compound constmction to resist fatigue, have an inner liner of AISI 410 or austenitic stainless steel. The associated small bore pipework and fittings used to transfer the initiator to the sparger are usually made of cold worked austenitic stainless steel. The required pumping capacity varies considerably from one process to another, but an initiator flow rate 0.5 L / min is more than sufficient to supply a single injection point in a reactor nominally rated for 40 t/d of polyethylene. 

Many polymers, including polyethylene, polypropylene, and nylons, do not dissolve in suitable casting solvents. In the laboratory, membranes can be made from such polymers by melt pressing, in which the polymer is sandwiched at high pressure between two heated plates. A pressure of 13.8—34.5 MPa (2000—5000 psi) is appHed for 0.5 to 5 minutes, at a plate temperature just above the melting point of the polymer. Melt forming is commonly used to make dense films for packaging appHcations, either by extmsion as a sheet from a die or as blown film. 

Similarly, the random introduction by copolymerization of stericaHy incompatible repeating unit B into chains of crystalline A reduces the crystalline melting point and degree of crystallinity. If is reduced to T, crystals cannot form. Isotactic polypropylene and linear polyethylene homopolymers are each highly crystalline plastics. However, a random 65% ethylene—35% propylene copolymer of the two, poly(ethylene- (9-prop5lene) is a completely amorphous ethylene—propylene mbber (EPR). On the other hand, block copolymers of the two, poly(ethylene- -prop5iene) of the same overall composition, are highly crystalline. X-ray studies of these materials reveal both the polyethylene lattice and the isotactic polypropylene lattice, as the different blocks crystallize in thek own lattices. 

At this point in the process, thermoplastic and chlorosulfonated polyethylene (CSPE) membranes are complete and are ready for packaging. In the case of ethylene—propylene—diene monomer (EPDM), the curing step occurs before the membrane is ready for packaging. The curing process is accomphshed by placing the membrane in a large vulcanizer where the material is heated under pressure to complete the cure. 

DistHlation is then used to separate the hydrocarbons into different products, including Hquid fuels and waxes with melting points ranging from about 45—106°C. Currently the waxes are produced in large volumes in South Africa and Malaysia, with an estimated 12,000—14,000 t consumed in the United States in 1994. Uses are similar to those for polyethylene waxes, including hot-melt adhesives and additives for inks and coatings. 

Fig. 5. Solubility coefficient at 30°C versus boiling point of ester in a low density polyethylene film (18). For unit conversion see equation 6.

HDPE melts at about 135°C, is over 90% crystalline, and is quite linear, with more than 100 ethylene units per side chain. It is harder and more rigid than low density polyethylene and has a higher melting point, tensile strength, and heat-defiection temperature. The molecular weight distribution can be varied considerably with consequent changes in properties. Typically, polymers of high density polyethylene are more difficult to process than those of low density polyethylene. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

Water-soluble polymers and polyelectrolytes (e.g., polyethylene glycol, polyethylene imine polyacrylic acid) have been used success-hilly in protein precipitations, and there has been some success in affinity precipitations wherein appropriate ligands attached to polymers can couple with the target proteins to enhance their aggregation. Protein precipitation can also be achieved using pH adjustment, since proteins generally exhibit their lowest solubility at their isoelectric point. Temperature variations at constant salt concentration allow for frac tional precipitation of proteins. 

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