HP LDPE

PE produced by a high-pressure polymerization process (pressure 1000-3000 atm) using a free radical initiator is a highly branched material that contains both LCBs and SCBs. The polymer so produced is a low-density material (density up to about 0.925 g/cc) and is known as high-pressure low-density PE (HP LDPE). The LCBs are formed via intermolecular hydrogen transfer [19], whereas SCBs are formed by intramolecular hydrogen abstraction [16]. 

Various fractionation methods have been used to study the heterogeneity of LCBs and SCBs of PEs [13]. The average values of LCBs in the commercial grades of HP LDPE range from 0.07-12.9 per 1000 C atoms [13,20,21], and their concentration increases with the increase in MW [15]. The content of SCBs (mainly from 1-6 carbon atoms) ranges from 8.5-22.4 per 1000 C atoms in HP LDPE [22,23]. It is reported that paired and/or branched branches are also found in HP LDPE [23]. 

The SCB distribution (SCBD) has been extensively studied by fractionation based on compositional difference as well as molecular size. The analysis by cross fractionation, which involves stepwise separation of the molecules on the basis of composition and molecular size, has provided information of inter- and intramolecular SCBD in much detail. The temperature-rising elution fractionation (TREE) method, which separates polymer molecules according to their composition, has been used for HP LDPE it has been found that SCB composition is more or less uniform [24,25]. It can be observed from the appearance of only one melt endotherm peak in the analysis by differential scanning calorimetry (DSC) (Fig. 1) [26]. Wild et al. [27] reported that HP LDPE prepared by tubular reactor exhibits broader SCBD than that prepared by an autoclave reactor. The SCBD can also be varied by changing the polymerization conditions. From the cross fractionation of commercial HP LDPE samples, it has been found that low-MW species generally have more SCBs [13,24]. 

While comparing the shear thinning behavior of HP LDPE and LLDPE having identical MFI (2.0 g/10 min), the value of zero shear viscosity (170) of the former has been found higher than the latter despite the lower Mn... 

Figure 7 Effect of shearing history on swelling ratio for HP LDPE sample (A) Brabender worked at 190°C (A) solvent treated sample. Source Ref. 39.

Figure 10 shows that upon cessation of shear flow of the melt, shear stress relaxation of LLDPE is much faster than HP LDPE because of the faster reentangle-... 

In another study [31] it has been reported that haze of LLDPE is mainly a consequence of light scattered by the spherulites. Thus, origin of poor clarity is not the same in LLDPE as in HP LDPE. In HP LDPE the haze is produced by surface defects coming from processing rheology [48,54,55]. Processing variables, therefore, do not have the same effect on LLDPE as they do on HP LDPE. 

In order to determine the branching structure factor e, Foster ( ) studied a large qroup of high pressure low density polyethylene resins (HP-LDPE). Using the MWBD method, he calculated the whole polymer number average number of branch points per JOOO carbon atoms from SEC data as a function of e. Then the Xfj values were compared with those obtained by nMR. 

An example of this method of determining e is shown in Figure 3 where the Am has been calculated as a function of e for three HP-LDPE resins, designated LDPE A, B, and C. LDPE A was produced at the highest conversion and LDPE C at the lowest conversion. The SEC data used were obtained at 140°C in 1,2,4-trichlorobenzene. The Mark-Houwink coefficients used for linear polyethylene were K = 5.1 x 10-4 dl/gm and a = 0.706. The 13c NMR Xm values are indic ted by open triangles. 

Dr. E. S. Hsi (Union Carbide Corporation, Bound Brook, New Jersey) provided the 13c NMR of the three HP-LDPE s studied. 

The results of the exergy analysis of the HP LDPE process are given in Figure... 

Grassmann diagram of the HP LDPE process. Available work, lost work, and potential steam bonus streams are given as a percentage of the total input. 

Pie chart of main exergy losses in the HP LDPE process. 

Table 2. Blown Film Property Comparison between HP-LDPE and LLDPE...

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