High-pressure homogenization, freeing

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

Many of the disadvantages of ram extrusion are eliminated in hydrostatic extruaon, where the piston is replaced by high pressure fluid. As shown in Fig. 15, the billet stands clear of the cylinder walls, so that this element of friction is eliminated. The pressure transmitting fluid also lubricates the interface between the die and the billet, so that the deformation is essentially plug-flow (i.e. an extensional deformation, identical to drawing polymer with a free surface). Homogeneous oriented sections are therefore produced. A small haul-off force is applied to provide control of the extrusion process, and this also serves to ensure that the extrudates are straight. 

Late in the 1960s, it became apparent that catalyzed lacquers, the hardest, most durable nitrocellulose lacquers commercially available for coating wood, had less resistance to alcohol, cosmetics, and chemicals than did the best quality alkyd-amine compositions or high-pressure melamine overlays. This situation was soon remedied by the development of nitrocellulose-urethane "super lacquers" based on nitrocellulose-isocyanate prepolymers (16). Nitrocellulose as manufactured contains free hydroxyl groups that can provide sites for true cross-linking with an isocyanate-bonded homogeneous coating. Laboratory studies confirmed the theoretical reaction ... 

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