HDPE SPIRAL PIPE INFORMATION PE100
PE100 High Density Polyethylene Materials for producing Spiral Wound Pipes
1 The discovery and development of polyethylene for pipe applications
The story of polyethylene (PE) really starts in 1933 with its accidental discovery by two scientists working for Imperial Chemical Industries (ICI) at Northwich, England, when a mixture of ethylene and benzaldehyde waas put under a pressure of serveral hundred atmospheres. The reaction that occured was actually initiated by oxygen contamination of the apparatus and did not involve the benzaldehyde, a fact that did not become apperent for another three years. after a great deal of trial and error, the discovery did however lead to the first commercial production of Low Density Polyethylene (LDPE) in 1939. once it was discovered that LDPE was an excellent cable insulator, particulary for high frequency radio and radar equipment, it was classified as a ‘secret’ material and commercial production suspended for the period of the 2nd World War. It did however continue to be produce in the UK and from 1942 was produced in the USA. after the war PE was declassified and commercial production resumed, though the extreme temperatures and pressures involve in the production process meant that it was an expensive material to produce.
During the late 1940s and early 1950s serveral groups were trying to develop catalysts that would promote ethylene polymerisation and hence allow polyethylene to be produced under less demmanding condition. The first was produce by Philips Petroleum of the USA in 1951, but the big breakthrough came with the catalytic system developed by German scientist Dr Karl Ziegler of the Max Plank Institute in 1953. This allowed High Density Polyethylene (HDPE) to be produced at relatively low pressures and temperatures and hence at a lower cost. at the same time an Italian scientist, Giuilo Natta discovered how to use a similar process to produce other materials such as polypropylene. this family of catalysts is therefore known as Ziegler-Natta catalysts and the two scientists shared a Noble prize for their work in 1963.
The early LDPE materials were very flexible, but the manufacturing process led to them having a branched amorphous structure, which resulted in their low density and hence weak physical properties. By contrast, the early HDPE materials had little if any branching and hence they were able to form a more dense structure with greater areas of crystallinity. whilst this gave them a reasonably high strengh it also meant that they were relatively brittle material from which to produce pipes. the answer was to introduce very limited quantities of a co-monomer such as butene in the polymerisation process to allow degree of branching. Depending with better flexibility and higher stress crack resistance, together with Medium Density Polyethylene (MDPE) which combined the properties of the two earlier materials.
Further catalyst development in the 1970s and1980s allowed the development of other polyethylene materials such as Linear Low Density Polyethylene (LLDPE). the diagrams shown in figure 1 present a simplified structure of the different types of PE. The first pipes were produced using the new HDPE materials in 1954 and the first installation was in 1955. continous development of the materials eventually led to HDPEs that were predicted to be able to sustain a hoop stress of at least 6.3 MPa (N/mm²) for a period of 50 years at a constant temperature of 20°C.
These were classified as PE63 materials and are generally considered to represent the 1st generation of practical PE pipe materials. Advances in understanding how to improve materials properties gradually led to the development of PE80 HDPE materials in the 1970s and that are considered as the 2nd generation in the development of PE pipe grades.
The next breakthrough in polymerisation came in the development of bimodal technology in the 1980s, an example of wich is shown i figure 2. The two stage process allows the polymer structure to contain both shorter chains, wich provide lubrication during processing and then from strong crystalline areas when the material cools, together with long chains that have limited branches and which provide resistance to slow crack growth and rapid crack propagation. this 3rd generation of PE pipe materials can sustain hoop stresses in excess of 10MPa and hence are classified as PE100 materials.
In recent years the continued development of HDPE has led to a 4th generation of materials that have very high resistance to slow crack growth, enabling PE100 pipes to installed with more confidence in challenging conditions such as when laid in rocky areas or installed using a variety of trenchless technologies. Figure 3 gives an overview of the development of PE pipe grades from the 1950s.
Most of these new materials, which are referred to as High Stress Crack Resistant (HSCR) PE100 or PE100-RC materials, use a hexene co-monomer, rather than butane. Under the correct polymerisation this leads to a PE molecular structure that has longer side branches. A higher level of the co-monomer also results in a greater number of these side branches. These two differences result in the tie molecules, that are found in the amorphous area of the polymer and which connect the crystalline areas together, having a much greater resistance to the propagation and propagation of any cracks. Figure 4 shows an example of the hydrostatic test undertaken to determine the long-therm hoopstress that a PE pipe material can sustain.
Whilst the development of PE grades has focused on pressure pipe applications, those same improvements in material properties and particularly the driver for materials that have a good balance between strength, toughness and durability have also lead to materials that are well suited to the production of gravity pipes. the next section will go on to describe, in detail, this balance of properties.
2 Key material properties
2.1 Introduction
Plastic gravity pipes are usually specified by their internal diameter and the required stiffness class, which is normally given as a SN (Nominal Stiffness) class measured in kN/m2. The stiffness of the pipe is determined by the stiffness or elastic modulus of the material used in its manufacture, together with the pipe’s structured profile. Spiral wound pipe systems are ideal for large diameter gravity drainage and seawarage networks, together with industrial applications such seawater intakes and outfalls. The manufacture process is very flexible, enabling pipes to be custome designed over a wide range of diameters and stiffness classes.
Both polyethylene (PE) and high modulus polypropylene (PP-HM) materials can be used to manufacture spiral wound pipes, with PP-HM materials bieng increasingly chosen for larger diameter and higher performance pipes. PE is more flexible (has a lower elastic modulus) than PP and therefore pipes of the same size and stiffness will be heavier and it is easier to join using the butt, electrofusion and extrusion welding and has a better impact performance at low temperatures.
2.2 Long term durability
Many low cost structured wall pipes and conduits are produced using HDPE materials that are actually meant for blow moulding applications. Whilst these grades may have a higher elastic modulus than PE100 materials, they are designed for manufacturing items that will have a relatively short life span. Hence they lack the durability of PE100m which has been developed for producing pressure pipes that will have a life of at least 50years. Table 1 provides a brief comparasion of the different material properties.
With reagards to durability, one key difference is the degree of protection from weathering, which principally comprieses of the attack by the UV rays in sunlight. In the case of coloured pipes this protection comprises of a UV stabiliser and if the pipes are to be black in colour, the protection is provided by the black carbon itself. Unfortunately experience has shown that the addition of either a UV stabiliser or black carbon by the pipe manufacturer prior to the extrusion stage frequently results in these essential additives not being properly dispersedthroughtout the polyethylene matrix. this allows the UV rays in sunlight to attack the PE, when the pipes are bieng stored on site, cutting the long polymer chains and so therefore weakening the pipes even before they are installed. It is for this reason that the PE pressure pipe standard, ISO 4427, only allows the use of materials that are properly compounded at the resin producer’s plant.
Although the pipe will not be pressurized during operation, it will still have to withstand significant loads from the surrounding soil and any traffic passing over the pipeline. therefore long term properties such as such as the Environtmental Stress Crack Resistance (ESCR) and the creep modulus of the materials from which the pipes are produced are essential. As it can be seen in Table 1 the stress crack resistance of the pipa grade material, measured by ESCR or FNCT tests, is far higher than that of the blow moulding grade. this is largely down to the PE100 being a bimodal material with a broader molecular weight distribution.
With regards to creep modulus, such information is rarely available for blow moulding grades as they are not designed and tested for such applications. Whilst signigicanly lower than that of PP-HM materials, is quite sufficient provided more material is used in the pipe manufacture.
3 Conclusions
ISO 4427 compliant PE100 pipe materials have a long and successful track record of bieng to produce pressure pipes and they have been developed to the point where they have an excellent balance of strength, toughness and durability. It is this balance of properties that also make then well suitd for producing spiral wound gravity pipes.
source article :
Handbook on large plastic pipe
by KRAH COMMUNITY
Authors :
Günter Dreiling, Borealis Poliolephine GmbH, Austria
Andrew Wedgner, Borouge Pte.Ltd. United Arab Emirates