Polyethylene Fractionation

Polyethylene is commercially produced either by a high-pressure free-radical polymerization or by a Ziegler-Natta or Unipol catalytic process. The polyethylene produced in these processes typically has not only a distribution of molecular weights, but also a distribution in backbone structure or branch content. Therefore, it should be possible to fractionate the parent material with respect to either molecular weight or backbone structure although, as subsequently described, the experimental techniques used to carry out these two types of fractionations are quite different. Let us first consider the properties of polyethylene that fix the operating philosophy used to fractionate with respect to backbone structure. 

The primary structural feature that determines crystallinity is the concentration of short chain branches along the polymer backbone. These short chain branches disrupt the local ordering of the chains and, thus, reduce the degree of crystallinity in the material. The melting point (T, eit) of the polymer is directly related to the amount of short chain branches in the material. Regardless of the supercritical fluid used, exceptionally low solubilities are found at temperatures below Tmeit- At temperatures above the strong 

The data in tables 9.1 and 9.2 were taken from Watkins et al. (1991) who demonstrate the impact of the crystallization boundary on the fractionation behavior of high-density polyethylene (HOPE). The second critical end point temperature for this sample of HOPE in propane is approximately 125°C. The data in table 9.3 show that HDPE can be selectively fractionated with respect to molecular weight across its entire molecular weight distribution using an isothermally increasing pressure profile in the liquid-gas region of the phase diagram. 

The data in table 9.2 show that HDPE cannot be efficiently fractionated when the HDPE is a solid at extraction conditions. Less than 5wt% of the HDPE charged to the column could be dissolved at pressure levels as high as 663 bar (9,615 psia) at a temperature below the second critical end point of the polymer. The material that was extracted in this fractionation was likely amorphous low molecular weight polymer present at or near the surface of the 

LLDPE Fractionation with Respect to Molecular Weight and Backbone Structure 

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Some by-product polyethylene waxes have been recently introduced. The feedstock for these materials are mixtures of low molecular weight polyethylene fractions and solvent, generaHy hexane, produced in making polyethylene plastic resin. The solvent is stripped from the mixture, and the residual material offered as polyethylene wax. The products generaHy have a wider molecular weight distribution than the polyethylene waxes synthesised directly, and are offered to markets able to tolerate that characteristic. Some of the by-product polyethylene waxes are distHled under vacuum to obtain a narrower molecular weight distribution. 

Figure 14 Most characteristic small-angle laser light scattering patterns (Hv) observed with linear polyethylene fractions. Reproduced by permission of The Royal Society of Chemistry from Ref. [224].

Figure 15 Morphological map of linear polyethylene fractions. Plot of molecular weight against crystallization temperature. The types of supermolecular structures are represented by symbols. Patterns a, b and c represent spherulitic structures with deteriorating order from a to c. Patterns g and d represent rods or sheet-like structures whose breadth is comparable to their length g or display a different aspect ratio d. Pattern h represents randomly oriented lamellae. Neither h nor g patterns have azimuthal dependence of the scattering. Reproduced with permission from Ref. [223]. Copyright 1981 American Chemical Society. (See Ref. [223] for full details.) Note the pattern a is actually located as o in the figure this was an error on the original.

Figure 20 Normalized frequency distributions of crystallite thickness for linear polyethylene fraction, Mn = 76,700, Mw = 80,800 crystallized at 118 °C ( ), electron micrograph histogram ...

Figure 21 Plot of thickness values as a function of molecular weight for linear polyethylene fractions quenched to —78°C. (A), crystallite thickness, Lc (O), interlamellar thickness, La ( ), interfacial thickness, Lb. Reprinted with permission from Ref. [277]. Copyright 1990 American Chemical Society.

Table 1 Mn, Mw and molecular weight distribution of polyethylene fractions fractionated...

Melted (150°C) Poly wax 3000 (PW3000, polyethylene fraction with average molecular weight of 3,000, Baker Petrolite, Inc.) or Ethylflo 164 hydrocarbon oil... 

Polywax 500 and 655 (polyethylene fraction with average molecular weights of 500 and 655, respectively), purchased from Baker Petrolite, Inc., was used to prepare simulated FT wax (i.e., solvent) for the evaluation of flltration performance with and without the presence of aliphatic alcohol and mono-olefin in the FT wax. 1-dodecanol (ACS reagent, > 98%, Sigma-Aldrich, Inc.) and 1-hexadecene (GC standard grade reagent, > 99.8%, Sigma-Aldrich, Inc.) were used as model... 

Moore and Millns (40) and also Hama and co-workers (42) have attempted to overcome this difficulty by estimating the weight average value w from the z-average value using an assumed MWD of breadth indicated by measurements of Mw and Mn on the (polyethylene) fractions they studied, but their procedure involves the assumption that is proportional to M, which may not be accurate for branched molecules. However, their procedure is probably the best currently available for the estimation of the weight-average value of s2. 

It is noticed that the -values are increasing in the same order as the ratios w = MJMn for all polymers. Due to the omission of the said extrapolation, actual -values will probably be somewhat smaller (except for PS III and the polyethylene fraction, which is discussed below). 

Fig. 3.8. Extinction angles % vs. reduced shear rate for solutions of a high density polyethylene fraction (Dow Chem. Corp.) in transdecalin at 160° C (75). The concentrations are indicated near the curves in g/100 cm. Open and closed symbols indicate repeat measurements. The dotted line gives the extinction angle vs. at zero concentration, as obtained by linear extrapolation at several...

Figure 1. Calibration curve for the gel-permeation chromatographic tracing obtained with polyethylene fractions, and n-C9UH190 and n-C36H7i...

In addition to these studies on the branched polyethylene, fractions of linear polyethylene prepared by large-scale gel-permeation chromatography are being characterized for certification in the near future. These should be useful for gel-permeation chromatography calibration. We also expect them to be particularly valuable in dilute solution, crystallization, and rheological studies. 

NMWD Sample. The MN values of this sample are in the range 10,000 to 30,000. However, 30,200 obtained from the infrared analysis appears to be too high. The value is based on the one-double-bond-per-molecule assumption. It automatically assumes the presence of branches. If no branches are present, MN is calculated as 18,500, which is more in line with other data. The GPC MN values are between 10,700 and 17,500. The lower value is the result of including the low molecular weight peak between 10 and 35 A. Probably this peak is not of polyethylene fraction because the lowest possible MN estimated from the infrared is 18,500. Mn by osmometry may be too large for the same reason stated before. The correct MN for this sample is probably 13,600-18,500. 

A universal calibration curve was established by plotting the product of the limiting viscosity numbers and molecular weight, Mw[iy], vs. the elution volume, EV, for a variety of characterized polymers. The major usefulness of the universal calibration curve was to validate individual molecular-weight values and to provide extended molecular-weight calibration at the ends of the calibration curve where fractions of narrow dispersion of the polymer being analyzed are not available. The calibration curve was monitored daily with polystyrene fractions certified by Pressure Chemicals. The relationship between the polyethylene fractions and polystyrene fractions was determined using the universal calibration curve. 

Crystallisation at higher temperatures (degree of super-cooling <17.5 °C) of linear polyethylene fractions of molar masses between 18,000 and... 

Linear polyethylene fractions (Mw/Mn= 1.26-1.81) in the molar mass range 119,000 to 800,000 g mol 1 also crystallise at high temperatures in spheru-lites Hoffman et al. [45] refer to these as irregular spherulites. 

Fig. 20 Transmission electron micrograph of permanganic-etched linear polyethylene fraction crystallised at 130.4 °C for 27 days. Courtesy of D.C. Bassett. From [46] with permission from the Royal Society of London, UK...

T9 —-A light-scattering study of low pressure polyethylene fractions. J. Polymer Sci. 36, 287 (1959). 

Fig. 4. Distribution of CHj trans lengths for polyethylene fractions fast quenched into n-pentane...

Fig. 14. Dependence of crystal thickness as a function of crystallisation time at constant temperature for linear polyethylene fraction...

Fig. 20. Detail in low molecular weight (M = 5.6x10 ) polyethylene fraction (T = 125 °C)...

A detailed analysis of the temperature dependent solid-state C MAS NMR spectrum of a low molecular weight polyethylene fraction that was isother-mally crystallized in extended chain forms is reported. ... 

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