Optimised Properties. PE-HD Compounds for Pipe Extrusion.
Apr 28, 2006
As a compound for pipe extrusion, PE-HD has experienced 50 years of continuous success both in terms of ever increasing production quantities and in terms of constant product improvement - one example being the development of bimodal PE-HD. With a multimodal PE-HD the processing characteristics can now be improved in particular.
- crystalline structures for high rigidity and negligible tendency to creep, and
- non-crystalline,amorphous structures for extreme toughness and resistance to stress cracking.
If the polymerisation process is divided into two stages, the co-monomer can be more effectively added during the second stage . In this way, short, unbranched molecular chains are polymerised in the first stage,while in the second stage chains of extremely high molecular weight are polymerised, as they incorporate the copolymer and therefore contain the desired short-chain branchings.
This bimodal PE-HD manifests a molecular weight distribution with two peaks and,given the same co-monomer content, a higher density than that of unimodal PE-HD. In these polymers, which are known as intrinsic polymer alloys or reactor blends , the short chains are primarily of crystalline structure, resulting in high rigidity and negligible tendency to creep,while the long chains with the short chain branchings form an amorphous structure and at the same time serve as tie molecules to hold the crystalline structures together (Fig. 1) . This dual function of the high-molecular chains makes for high values in terms of toughness, stress cracking resistance and notch resistance.
|Hostalen type||Typical properties||Pipe applications|
|CRP 100 Black||outstanding balance of rigidity and toughness, good processability||listed as a PE 100+ material; pressure pipes for supply and disposal (gas, water, sewage, chemicals), sheets, profiles, fittings|
|CRP 100 Blue||outstanding balance of rigidity and toughness, good processability||listed as a PE 100+ material; pressure pipes for drinking water supply|
|GM 5010 T3 Black||good balance of rigidity and toughness, good processablility||listed as a PE 80+ material; pressure pipes for supply and disposal (gas, water, sewage, chemicals), Sheets, profiles, fittings|
|GM 5010 T3 Natural||good balance of rigidity and toughness, good processablility||sheets and other semi-finished materials for tanks and container, wear and corrosion resistant claddings|
Moreover, the resistance to rapid crack propagation is so high that during the S4 test no rapid crack propagation took place even at pressures as high as 20 bar (Fig. 3; test performed at 0°C on pipes of 110 mm nominal diameter) . Added to this is the considerably reduced susceptibility to stress cracking. Indeed, this has been reduced to such an extent that no stress cracking was observed at all in the creep tests performed in accordance with ISO 1167 (cf. Fig. 2). It was not surprising that Hostalen CRP 100 Black, the high-performance compound for pipe extrusion, was chosen as one of the reference materials for tightening the requirements of the pipe materials listed with the PE 100+ Association, Apeldoorn/Netherlands (website: http://www.pe100plus.net).
Regarding long-term stability, i. e. resistance to thermo-oxidative degradation, it could be shown that the Oxygen Induction Time (OIT),which serves as the measure of long-term stability, furnishes only a bottom limit, that is to say, the actual duration of resistance may be considerably longer than the OIT indicates.The Swedish firm of Bodycote Polymer, Nyköping, has carried out long-term failure tests under internal hydrostatic pressure at a test temperature of 80°C, which show that thermo- oxidative degradation in the case of pipes manufactured from Hostalen CRP 100 Black occurs after 20000 hours (approx. 2.3 years),while in the case ofHostalen CRP 100 Blue thermo-oxidative degradation does not take place until after 50000 hours (approx.5.7years), although the OIT value is virtually the same for both grades (Fig. 4). The reason for this lies in the fact that the HALS stabilizer used in the blue polymer as a light stabilizer does not influence the OIT value, but through synergistic interaction with the primary antioxidants very likely contributes to an improvement in long-term stability .
Continuous product improvement has resulted in the modification of individual parameters within the existing cascade polymerisation process and, without necessitating any changes to the basic processing sequence, in the development of a new process, the "Advanced Cascade Process – ACP", in which specific partial processes take place in reactors arranged in tandem (Fig. 5), the resultant product being a multimodal PE-HD .This cascade process permits incorporation of the important, property-influencing comonomers in the high-molecular fraction much more effectively and systematically than before . The co-monomer, the compounding technology and the additive system remain unchanged, as do the important material constants, e. g.the melt flow index MFR190/5 and the density.
With the Advanced Cascade Process, it is possible to jointly optimise environmental stress cracking resistance (ESCR), rapid crack growth resistance (creep),uneven wall thickness distribution in large, thick-walled pipes (sagging) and processability (Fig. 6), although these four requirements for high-quality PE-HD compounds for pipe extrusion at first seem to be altogether contradictory from the point of view of microstructure and molecular structure.
The decisive factor for reducing sagging and improving processability is the rithmic scale), the differences in effect are considerable: the lower the shear rate, the higher the viscosity – and hence the strength – of the melt. As the cooling of large, thick-walled pipes takes a relatively long time, a low viscosity melt tends to gravitate, particularly in the more or less vertical, inside walls of the pipe where the cooling is least effective, with the result that, after calibration, the originally uniform cross section of the pipe manifests a thickening of the pipe wall along its underside. The higher strength of the melt can reduce this effect to such a degree that it is of virtually no further significance.
- Production tolerances when extruding thick-walled pipes are significantly narrower.
- Melt temperature can be reduced by up to 10 K and melt pressure by up to 20 bar.
- The required extruder torque is lower; power input is reduced by up to 8 %.
Thus Hostalen CRP 100 Black, the high performance compound for pipe extrusion, displays an optimum balance of the most important material properties for pipe applications.
 Glenz, W.: High-density Polyethylene (PE-HD). Kunststoffe 94 (2004) 10, pp. 58–61
 Richard, K.; Diedrich, G.: Rohre aus Niederdruckpolyäthylen – Eigenschaften und Erprobung in Labor und Praxis. Kunststoffe 46 (1956) 5, pp. 183–190
 Richard, K.; Diedrich, G.: Standfestigkeitseigenschaften von einigen Hochpolymeren. Kunststoffe 45 (1955) 10, pp. 429–433
 Böhm, L.: The Ethylene Polymerization with Ziegler Catalysts: Fifty Years after the Discovery: Angew. Chem. Int. Ed. 2003, 42, pp. 5010–5030
 Beer, G. et al.: High-density Polyethylene (PE-HD). Kunststoffe plast europe 92 (2002) 10, pp. 10–13
 Jansen, N.; Lecht, R.; Wernicke, K.: What Can PE 100 Do? Kunststoffe plast europe 86 (1996) 8, pp. 15–17
 Schulte, U.: 100 Years' Service Life. Kunststoffe plast europe 87 (1997) 2, pp. 25–26
 Schulte, U.: Rohrleitungssysteme aus Kunststoffen – unbegrenzte Lebensdauer? Oldenburger Rohrleitungstage, 4th–5th Feb. 1999
 Schulte, U.: HDPE pipes are more resistant to oxidation than the OIT indicates, lecture at plastics pipes XII, Baveno/Italy, April 2004
Dipl.-Ing. Schulte, born 1948, is Head of Technical Service and Application Development, Business Unit Pipes, at Basell Polyolefine GmbH, Frankfurt am Main/Germany.
Increased capacity with new plant in Wesseling
There is an ever-growing demand for compounds for pipe extrusion and the recognition that multimodal PE-HD can also improve processing and functional properties when used for film extrusion, blow moulding and special injection moulding applications. Basell has installed a large-capacity production line for multimodal PE-HD at its site in Wesseling/Germany. With a capacity of 320 000 t/a, the new production line, which was started-up in the fourth quarter of 2004, represents a scale-up of the ACP production line at Basell’s Frankfurt site, which is rated for 110 000 t/a. Everything else – the co-monomer used for the polymerisation process, the additive system and the type and quantity of carbon black or pigment concentrate – remain unchanged. This production line produces Basell’s entire range of PE-HD compounds for pipe extrusion (Table 1) and other ACP products for blow moulding applications. An additional line of the same type, built in Plock/Poland, for the Basell Orlen Joint Venture, will start production in the third quarter of 2005.
This article is kindly supported by Kunstoffe International: http://www.kunststoffe-international.com "First published in Kunststoffe international 1/2006"
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