Medium-density polyethylene processing

Its greatest deficiency is processability—it is hardly thermoplastic at all Hercules sells mill shapes which must be machined into 6nd products, while Formica has developed a continuous compactionsintering machine for producing it in sheet and laminate form (3). In addition, its modulus, strength, and heat distortion temperature are no better than conventional medium-density polyethylene. Bar, plate, and rod stock sell for 2 up, before machining. 

R. Ochoa, H.V. Woert, W.H. Lee, R. Subramanian, E. Kugler, and PC. Eklund, Catalytic degradation of medium density polyethylene over silica-alumina supports. Fuel Process. Technol., 49, 119 (1996). 

Applications To produce high density polyethylene (HDPE) and medium density polyethylene (MDPE) under low-pressure slurry process—"CX process."... 

Application The Borstar polyethylene (PE) process can produce a broad range of bimodal and unimodal linear-low-density polyethylenes (LL-DPE), medium-density polyethylenes (MDPE) and high-density polyethylenes (HOPE). The Borstar PE consists of a loop reactor and a gas-phase reactor In series. 

Application The UNIPOL Polyethylene (PE) Process produces the widest array of linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE) and high-density polyethylene (HOPE) having unimodal or bimodal molecular weight distribution (MWD) using a single, low-pressure, gas-phase reactor. 

Medium density polyethylene (MDPE), 0.925-0.940 g/cm, is sometimes listed as a separate category, but usually is regarded as the high density end of LDPE. It is somewhat stronger, stiffer, and less permeable than lower density LDPE. MDPE processes similarly to LDPE, though usually at slightly higher temperatures. 

The milling process can be a suitable method for producing medium-density polyethylene-multiwalled carbon nanotubes. 

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). 

The low vinyl acetate ethylene—vinyl acetate copolymers, ie, those containing 10—40 wt % vinyl acetate, are made by processes similar to those used to make low density polyethylene for which pressures are usually > 103 MPa (15,000 psi). A medium, ie, 45 wt % vinyl acetate copolymer with mbber-like properties is made by solution polymerisation in /-butyl alcohol at 34.5 MPa (5000 psi). The 70—95 wt % vinyl acetate emulsion copolymers are made in emulsion processes under ethylene pressures of 2.07—10.4 MPa (300—1500 psi). 

E-plastomers, particularly the high- and medium-density materials, have found extensive use in films [17]. They are valued for their excellent seal character which allows the formation of mechanically strong seals at relatively low temperatures compared to traditional low-density polyethylene (LDPE). In addition, these E-plastomers can be obtained in a range of crystallinities and softness. These higher-density materials are typically made in the blown-film process and are used for protective film covers and disposable bags. 

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