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Whereas a considerable increase of the failure time is observed for the blends involving the components of HBPE 2 and HBPE 3 with a relatively high octene content at an increasing content of these components in the blend, the failure time in the blend involving HBPE 1 clearly decreases with an increasing content of the component in the blend.
It becomes evident that the use of an HDPE with butene as a comonomer with its relatively good ESCR in blends with HBPE with a relatively high octene content may cause a considerable increase in failure times.
In blends of a very high-molecular HDPE and HBPE with a higher octene content (HDPE 4/HBPE 3 system), the good ESCR of the high-molecular and hardly short-chain branched HDPE 4 (Fig.
Figure 6 shows the dependence of the ESCR of the HDPE/HBPE blends on the octene content in the HBPE (expressed as the weight fraction [w.sub.octene]) for various blend compositions and for HBPE components in the melt flow index (MFI) range from 1 to 2.5 g [(10 min).sup.-1].
When a short-chain branched HDPE of a relatively low density with a good starting level of ESCR (HDPE 5) is used in systems with LLDPE or HBPE of low density, even a small increase of the tie-molecule density beyond 0.13 results in a strong increase in failure time.
In the intermediate tie-molecule density range from 0.10 to 0.13, the use of a short-chain branched HDPE with a relatively good ESCR (HDPE 5) in blends with HBPE with a low comonomer content and hence relatively high density (HBPE 1) may result in a decreasing tie-molecule density at an increasing HBPE content, which explains the decrease in failure times and hence in ESCR of the blend (Fig.
In addition, the portion of the long-chain branches in the HBPE must be taken into consideration for the HDPE/HBPE systems, which, when increased, will probably lead to an increase in the tie-molecule density and is attributable to the inclusion of longer polyolefin units with an end double bond resulting from transfer reactions in the specific catalyst system.
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