Polyethylene Properties

Polyethylene (PE)

Polyethylene materials are manufactured from natural gas derived feedstocks by two basic polymerisation processes.

The low pressure polymerisation process results in linear polymer chains with short side branches. Density modifications to the resultant polymer are made by varying the amount of comonomer used with the ethylene during the polymerisation process.

The high pressure polymerisation process results in polymer chains with more highly developed side branches. Density modifications to the resultant polymer are made by varying the temperatures and pressures used during the polymerisation process.

The physical properties of PE materials are specific to each grade or type, and can be modified by both variations in density, and in the molecular weight distribution. General physical properties are listed in the Table below.

A large number of grades of PE materials are used in pipe and fittings systems and the specific properties are tailored for the particular application. Advice can be obtained from Vinidex as to the most effective choice for each installation. The most general types of PE materials are as follows:

Low Density PE (LDPE)

LDPE has a highly branched chain structure with a combination of small and large side chains. The density of LDPE ranges between 910-940 kg/m3 and LDPE exhibits high flexibility and retention of properties at low temperatures.

The main use for LDPE in piping is in the micro irrigation or dripper tube applications with sizes up to 32 mm diameter.

LDPE materials may be modified with elastomers (rubber modified) to improve Environmental Stress Crack Resistance (ESCR) values in micro irrigation applications where pipes operate in exposed environments whilst carrying agricultural chemicals.

Linear Low Density PE (LLDPE)

LLDPE has a chain structure with little side branching and the resultant narrower molecular weight distribution results in improved ESCR and tensile properties when compared to LDPE materials. LLDPE materials may be used either as a single polymer or as a blend with LDPE, in micro irrigation applications to take advantage of the material flexibility.

PE80 and PE100

The first PE pipe material used in engineering applications was Type 50 High Density PE (HDPE) with a long term stress of 50MPa. Subsequently, Medium Density (MDPE) materials, with improved pipe properties when compared to the earlier high density materials, were used in pipes due to their improved flexibility, ductility, slow crack growth resistance and crack propagation resistance.

The second and third generation PE pipe materials currently in use may be either medium or high density materials and are now referred to by their Minimum Required Strength (MRS). PE80 pipe materials have an MRS of 8.0 MPa and PE100 materials have an MRS of 10.0 MPa. PE pipes are widely utilised in pressure and non-pressure applications such as water supply, sewerage, gas reticulation, small diameter pipe coils, travelling irrigator coils, electrical and communications conduits and mining and industrial applications

Material Classification and Stress Regression

Hydrostatic Design Stress

The allowable hydrostatic design stress is based on the Minimum Required Strength (MRS) which is in turn obtained from stress regression curves.

Stress regression curves are developed from short and long term pressure testing of pipe specimens. As there is a linear relationship between the logarithm of the applied stress and the logarithm of time to failure, the test points are plotted and extrapolated to an arbitrarily chosen 50 year point.

In some cases, especially at higher temperatures, there is a sudden change in slope of the regression curve, known as the ‘knee’. The knee, as illustrated in the Figure below represents the transition from ductile failure mode to brittle failure mode.

The relationship between the curves for different test temperatures enables prediction of the position of the knee at 20°C, based on a known position at elevated temperature. This in turn enables prediction of ductile life at 20°C.

The value of the predicted hoop stress (97.5% lower prediction limit) is determined at the 50 year point. Based on this, the PE compound is classified as PE 80 or PE 100 in accordance with the Minimum Required Strength (MRS) of the material, i.e. 8.0 or 10.0 MPa.

The hydrostatic design stress is obtained by application of a factor, not less than 1.25, to the MRS value. It is emphasised that stress regression curves form a design basis only, and do not predict system life.

Stress Regression Curves

To design a pipe with the required thickness for a given pressure and diameter, for example, the following formula applies:

σ = MRS/C

σ = P(D-e)/2e

where:

Typical Properties of Polyethylene Pipes and Fittings Compounds

Material Performance Aspects of PE

Abrasion Resistance

The transmission of solids in either liquid or gaseous carriers in PE pipelines results in abrasion of the internal pipe walls, especially at points of high turbulence such as bends or junctions. The high resistance to abrasion, flexibility, light weight, and robustness of Vinidex PE pipes, have led to their widespread use in applications such as transportation of slurries and mine tailings. Abrasion occurs as a result of friction between the pipe wall and the transported particles. The actual amount and rate of abrasion of the pipe wall is determined by a combination of:

• the specific gravity of the solids
• the solids content in the slurry
• solid particle shape, hardness and size
• fluid velocity
• PE pipe material grade

The interaction of these parameters means that any prediction of the rate of abrasion wear can only proceed where testing of wear rates has been performed on the specific slurry under the proposed operational conditions. Under varying test conditions the relative ranking of different pipe materials may change, and where possible testing should be performed.

In general terms, PE pipes have superior abrasion resistance to steel, ductile iron, FRP, asbestos and fibre reinforced cement pipes, providing a more cost effective solution for abrasive slurry installations. Laboratory test programs have been performed in the UK, Germany and USA to obtain relative wear comparisons for various materials using sliding and rotating pipe surfaces. The results of test programs using the Darmstadt (Germany) method of Kirschmer and reported by Meldt (Hoechst AG) for a slurry of quartz sand/ gravel water with a solids content 46% by volume and a flow velocity of 0.36m/s are shown in Figure 2.2.

These were performed across a range of materials and show the excellent abrasion resistance of PE pipe materials. Similarly, Boothroyde and Jacobs (BHRA PR 1448) 1 performed closed loop tests using iron ore slurry in a concentration range of 5 to 10% and ranked PE ahead of mild steel and asbestos cement in abrasion resistance. For most grades, the difference in abrasion resistance between MDPE and HDPE is not significant.

The design of fittings involving change of flow direction is critical in slurry lines. The lower the rate of change of direction, the lower the abrasion rate. For bends, a large centreline radius must be used. Where possible, a radius of at least 20 times the pipe diameter should be used, along with a long straight lead-in length containing no joints.

In practice, the effective lifetime of the PE pipeline can be increased by using demountable joints to periodically rotate the PE pipe sections to distribute the abrasion wear evenly around the circumference of the pipe.

Weathering

Weathering of plastics occurs by a process of surface degradation, or oxidation, due to a combined effect of ultra violet radiation, increased temperature, and moisture when pipes are stored in exposed locations.

All Vinidex PE pipe systems contain antioxidants, stabilisers and pigments to provide protection under Australian construction conditions. Black PE pipes contain carbon black which act as both a pigment and an ultra violet stabiliser, and these pipes require no additional protection for external storage and use.

Other colours such as white, blue, yellow or purple do not possess the same stability as the black pigmented systems and the period of exposure should be limited to two years for optimum retention of properties. With these colour systems the external surface oxidation layers develop at a faster rate than those in carbon black stabilised PE pipes. For exposure periods longer than two years, additional protection such as covering should be adopted.

Where non-black pipe is required for longer periods of exposed service, contact Vinidex for advice. For further information on weathering of PE pipes see Technical Note VX-TN-6C, Weathering of PE pipes.

Permeation

Permeation of PE pipe systems from external sources may occur when the surrounding soils are grossly contaminated. Permeation is complex and depends on factors such as soil type, concentration of contaminants, temperature, diffusion, pipe diameter and wall thickness and flow rate in the pipe. Organic compounds of the non polar, low molecular type are those which permeate most rapidly through the PE pipe walls. Accordingly, where materials such as aliphatic hydrocarbons, chlorinated hydrocarbons and alkylated benzenes are encountered in sufficiently high concentrations, consideration to impermeable ducting should be given. Where contamination is suspected, soil sampling should be performed and in the case of potable water transmission lines, protection to the PE pipes should be provided where contamination of significant concentration is found.

Biological Resistance

PE pipes may be subject to damage from biological sources such as ants or rodents. The resistance to attack is determined by the hardness of the PE used, the geometry of the PE surfaces, and the conditions of the installation. Small diameter irrigation applications using LDPE materials may be attacked by ants or termites due to the relatively thin wall sections and the hardness of the LDPE. In these instances the source of the ants should be treated by normal insecticide techniques. Both MDPE and HDPE material types have a higher hardness value than LDPE, and together with the thicker pipe wall sections used in PE63, PE80, and PE100 applications provide a generally resistant solution. In small diameter pipes, the thin wall sections may be damaged by termites in extreme cases. However PE does not constitute a food source and damage often ascribed to termite attack in PE has subsequently been found to be due to other sources of mechanical damage. PE pipe systems are generally unaffected by biological organisms in both land, and marine applications, and the paraffinic nature of the PE pipe surfaces retards the build up of marine growths in service.

Electrical Conductivity

Vinidex PE pipes are non-conductive and cannot be used for electrical earthing purposes or dissipating static electricity charges.

Where PE pipes are used to replace existing metal water pipes, the designer must consider any existing systems used for earthing or corrosion control purposes. In these cases, the appropriate electrical supply authority must be consulted to determine their requirements.

Static Electricity

Static electricity may be generated on the surface of PE pipe as a result of friction during handling, gas flow, squeeze off and purging.

In dry, dusty, or explosive atmospheres, potential generation of electricity must be evaluated, and safe static dissipation measures adopted to prevent any possibility of explosion.

Fire Rating

PE pipe systems will support combustion and as such are not suitable for use in fire rated zones in buildings without suitable protection. The individual fire rating indices for PE materials may be established by testing to the requirements of AS1530.

In multiple storey buildings PE systems penetrating floor cavities must be enclosed in fire rated service ducts appropriate to the Class of the building concerned or devices such as intumescent fire stoppers should be installed in accordance with the manufacturer’s instructions.