High-density polyethylene crystallinity

Second, in the early 1950s, Hogan and Bank at Phillips Petroleum Company, discovered (3,4) that ethylene could be catalyticaHy polymerized into a sohd plastic under more moderate conditions at a pressure of 3—4 MPa (435—580 psi) and temperature of 70—100°C, with a catalyst containing chromium oxide supported on siUca (Phillips catalysts). PE resins prepared with these catalysts are linear, highly crystalline polymers of a much higher density of 0.960—0.970 g/cnr (as opposed to 0.920—0.930 g/cnf for LDPE). These resins, or HDPE, are currentiy produced on a large scale, (see Olefin polymers, HIGH DENSITY POLYETHYLENE). 

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 Phillips-type catalyst can be used in solution polymerization, slurry polymerization, and gas-phase polymerization to produce both high density polyethylene homopolymers and copolymers with olefins such as 1-butene and 1-hexene. The less crystalline copolymers satisfy needs for materials with more suitable properties for certain uses that require greater toughness and flexibiUty, especially at low temperatures. 

Fig. 22.6. A schematic drawing of a largely crystalline polymer like high-density polyethylene. At the top the polymer has melted and the chain-folded segments hove unwound.

High Density Polyethylene (HDPE). This material has a density in the range 935-965 kg/m and is more crystalline than LDPE. It is also slightly more... 

Polyethylene has low density when polymerized at pressures 9,000 - 45,000 psi and high density when made with special catalysts at 250 - 500 psi. Low-density polyethylene softens 68 F lower than high-density polyethylene, which is more crystalline and stiffer. The rigidity characteristics and surface of high-density polyethylene are comparable with polystyrene. It feels like nylon, has a bursting strength three times that of low-density polyethylene, and withstands repeated exposure to 250 F, hence, it can be sterilized. 

High-density polyethylene is characterized by a higher crystallinity and higher melting temperature than LDPE due to the absence of branching. 

High density polyethylene produced by a low-pressure low-temperature process involving Ziegler-Natta catalysts. This creates low levels of branching and hence a high degree of crystallinity. 

Due to the low level of branching, there is little to hinder the crystallization of high density polyethylene. We routinely observe crystallinity levels in excess of 60%, which translate into densities ranging from approximately 0.94 to 0.97 g/cm3. High density polyethylene is the stiffest of all the polyethylene types. In some cases we incorporate small amounts of a comonomer, such as 1 -hexene, which reduce the crystallinity level. This improves toughness at the expense of stiffness. 

A major characteristic of the Phillips process chain polymerisation of ethylene is that it leads to very limited branching. The resulting polymer is thus highly linear and can reach high levels of crystallinity, hence high densities approaching 0.96-0.97. Such a polyethylene is known as HDPE for "High-density polyethylene". 

Ziegler-Natta polymerization leads to linear unbranched polyethylene, the so-called high density polyethylene (HDPE), which is denser, tougher and more crystalline. By copolymerization with other alkenes it is possible to obtain linear low density polyethylene (LEDPE) with better mechanical properties than LDPE. Blends of LLDPE and LDPE are used to combine the good final mechanical properties of LLDPE and the strength of LDPE in the molten state. 

Crystallinity. Is one of the key factors influencing properties. You can think of crystallinity in terms of how well a polymer fits in an imaginary pipe, as in Figure 22-6. Linear, straight chains are highly crystalline and fit very well. Bulky groups, coiled chains, and branched chains are not able to line up to fit in the pipe. They are amorphous, the opposite of crystalline. In a spectrum from totally amorphous, to almost totally crystalline, there is methyl methacrylate, polypropylene, low-density polyethylene, linear low-density polyethylene, high-density polyethylene, and nylon. 

R.R Paradkar, S.S. Sakhalkar, X. He and M.S. Ellison, Estimating crystallinity in high density polyethylene fibers using online Raman spectroscopy, J. Appl. Polym. Sci., 88, 545-549 (2003). 

Examples of crystalline polymers are nylons, cellulose, linear polyesters, and high-density polyethylene. Amorphous polymers are exemplified by poly(methyl methacrylate), polycarbonates, and low-density polyethylene. The student should think about why these structures promote more or less crystallinity in these examples. 

The polyethylene produced by radical polymerization is referred to as low-density polyethylene (LDPE) or high-pressure polyethylene to distinguish it from the polyethylene synthesized using coordination catalysts (Sec. 8-1 lb). The latter polyethylene is referred to as high-density polyethylene (HDPE) or low-pressure polyethylene. Low-density polyethylene is more highly branched (both short and long branches) than high-density polyethylene and is therefore lower in crystallinity (40-60% vs. 70-90%) and density (0.91-0.93 g cm 3 vs. 0.94-0.96 g cm-3). 

Crystalline polymers such as high-density polyethylene (hope), PP, PTFE, and polyoxymethylene (POM) exhibit somewhat higher X values than amorphous polymers such as low-density polyethylene (ldpe), atactic PS,... 

The data from this table illustrate the semicompatibility of the phase between isotactic polypropylene and the high density polyethylene with block copolymer without gross interference in the domain structure or the crystalline phases that exist in these TPR s. 

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