Optimised Properties. PE-HD Compounds for Pipe Extrusion.

With a market share of around 17 %, the manufacture of pipes for water, sewage and gas are the most important areas of application for high density polyethylene (PE-HD) in Europe [1]. As a pioneer in this field, the former Farbwerke Hoechst AG,Frankfurt am Main/Germany, already developed a special PE-HD for pipes 50 years ago [2] and introduced Hostalen GM 5010 onto the market only a short time later. Even during those early years,the company had already set up a "multiple specimen test bench for long-term failure tests with pipes under internal pressure at room temperature" [3]. Specimens produced from the first PE-HD extrusion compounds have been undergoing a continuous long-term failure test on a similar test bench since 1956 (title picture). These specimens will shortly be able to prove in real time, i. e. without acceleration through increased temperature, that even those pipes produced from the very first batches meet the high expectations achieved on their service life.

Naturally, further research and development work has considerably increased our knowledge about the material and its behaviour in practical application since its introduction into the market. A fact which is of decisive relevance to pipe applications is that partially crystalline PE-HD has

• crystalline structures for high rigidity and negligible tendency to creep, and
• non-crystalline,amorphous structures for extreme toughness and resistance to stress cracking.
   

In order to put this recognition into practice, it is necessary to produce an PE-HD with a corresponding microstructure. This is possible by means of a modified version of the low-pressure polymerisation process using Ziegler catalysts.

Bimodal Polyethylene – Rigidity and Toughness Combined
If the polymerisation process is divided into two stages, the co-monomer can be more effectively added during the second stage [4]. 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 [5], 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) [4]. This dual function of the high-molecular chains makes for high values in terms of toughness, stress cracking resistance and notch resistance.

The modified polymerisation process and the bimodal microstructure that can nowbe achieved with it enable the raw material producer to jointly optimise the previously mentioned conflicting functional properties. In the case of Hostalen CRP 100 Black, for example, it has been possible to increase its strength as extrapolated according to ISO/TR 9080 for a service life of 50 years at 20°C to over 11 MPa, as shown by creep tests performed within a range of temperatures from 20 to 80°C (Fig. 3) [6].Thus Hostalen CRP 100 Black significantly exceeds the minimum strength requirement of 10 MPa for performance class PE 100 by more than 10 %.

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) [6]. 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).

The extrapolation method used in accordance with ISO/TR further shows that pipes manufactured from bimodal Hostalen CRP 100 Black have a very long design life time (the value derived from the standard exceeds 100 years) [7, 8].

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 [9].

Basell commenced production of bimodal compounds for pipe extrusion in Frankfurt almost 10 years ago. Besides Hostalen CRP 100 Black and Blue, these compounds included Hostalen GM 5010 T3 Black, an PE-HD of performance class PE 80. This material is likewise distinguishedby highstrength, high stress cracking resistance and high notch resistance.

Improved Processing Properties

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 [5].This cascade process permits incorporation of the important, property-influencing comonomers in the high-molecular fraction much more effectively and systematically than before [4]. 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.

What is immediately comprehensible is the improvement in stress cracking resistance, as the effective incorporation of co-monomer in the long molecular chains increases toughness. When subjected to the full notch creep test (FNCT) in accordance with ISO 16770 (2 % wetting agent solution, 80°C, 4 MPa tensile stress), ACP products achieve almost double the values obtained previously.

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.

Conversely, the high shear rates which occur during the extrusion process lower the viscosity of the melt in the case of ACP products.This improves the processability and permits a lowering of the necessary melt temperature, in turn reducing energy consumption and diminishing the risk of sagging. Pipe manufacturers have meanwhile confirmed these advantages when processing ACP products on large extrusion lines:

• 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 %.

Moreover, the superior swelling characteristics of ACP products have a positive effect on processing and on the surface quality of the pipe. It reduces the risk of dead spots in the extrusion die outlet where melt stagnation and fracture can occur. Plate-out on the inner walls of the die and the concomitant deterioration in surface finish are largely eliminated.
 

The improvements achieved with the Advanced Cascade Process are summarized in the spider diagram in Fig. 8. In terms of long-term hydrostatic strength and fracture toughness, the ACP-produced Hostalen CRP 100 Black leads the field, while for the other properties its values are between 92 and 98 % of the respective best result.

 

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.




References
[1] Glenz, W.: High-density Polyethylene (PE-HD). Kunststoffe 94 (2004) 10, pp. 58–61

[2] Richard, K.; Diedrich, G.: Rohre aus Niederdruckpolyäthylen – Eigenschaften und Erprobung in Labor und Praxis. Kunststoffe 46 (1956) 5, pp. 183–190

[3] Richard, K.; Diedrich, G.: Standfestigkeitseigenschaften von einigen Hochpolymeren. Kunststoffe 45 (1955) 10, pp. 429–433

[4] Böhm, L.: The Ethylene Polymerization with Ziegler Catalysts: Fifty Years after the Discovery: Angew. Chem. Int. Ed. 2003, 42, pp. 5010–5030

[5] Beer, G. et al.: High-density Polyethylene (PE-HD). Kunststoffe plast europe 92 (2002) 10, pp. 10–13

[6] Jansen, N.; Lecht, R.; Wernicke, K.: What Can PE 100 Do? Kunststoffe plast europe 86 (1996) 8, pp. 15–17

[7] Schulte, U.: 100 Years' Service Life. Kunststoffe plast europe 87 (1997) 2, pp. 25–26

[8] Schulte, U.: Rohrleitungssysteme aus Kunststoffen – unbegrenzte Lebensdauer? Oldenburger Rohrleitungstage, 4th–5th Feb. 1999

[9] Schulte, U.: HDPE pipes are more resistant to oxidation than the OIT indicates, lecture at plastics pipes XII, Baveno/Italy, April 2004

The Author
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"