Posted by on 17 December 2009 | 0 Comments

When new materials haven't been in existence long enough to demonstrate their longevity in the field, producers need to use other means to reassure specifiers about the design-life of the materials they choose for infrastructure solutions. Here plastics specialist Dr Vasilios Samaras explains how he can demonstrate the 100-year plus design life of high-density polyethylene.

Over the centuries mankind has worked hard to develop synthetic materials that offered benefits not found in natural products around him. The first ever synthetic plastic was cellulose nitrate, discovered in the middle of the 19th century. Plastics started to develop in earnest throughout the 1920s and were used extensively by the military during the Second World War.

Nowadays, plastics are found everywhere in society and are often used to replace traditional materials such as wood, stone or metal. Of all plastic materials, high-density polyethylene (HDPE) manifests the strongest growth in usage, especially in the pipe industry. In Europe alone, approximately 1.3 million tonnes of HDPE was processed into pipes last year. In general terms, HDPE pipes provide designers, developers and contractors with a reliable, durable and cost-effective solution for a wide range of applications including gas, municipal, industrial, storm water attenuation, mining, landfill and electrical and communications duct applications.

HDPE pipes are also effective for above-ground, trenchless, floating and marine installations. In addition to the operational advantages offered, there are many cost benefits, including low maintenance and ease of installation, compared to traditional materials. This combination of assets has made plastic pipe a very competitive product. A question often raised, especially by traditional pipe manufacturers, is: How can you prove the life-span of 100 years for HDPE pipes when they have only been in existence since the 1950s? One wonders why this should be raised at all, given that, in this age of green awareness, environmental experts quite confidently predict that plastic carrier bags won’t degrade for up to 300 years.

Whilst this may be held as an environmental negative in that case, within the arena of pipes, it becomes a positive. However, the longevity of HDPE pipes can be proven with scientific research, as the following will show. Only two years after Prof. Karl Ziegler, succeeded in polymerising ethylene in the presence of organo-metallic catalyst blends under mild pressure and temperature conditions, the first regular HDPE production plant, with a capacity of 200 tonnes per month, was already in service. For this discovery Karl Ziegler, together with the Italian chemist Giulio Natta, was awarded the Nobel Prize for Chemistry in 1963. It was the beginning of a very successful story - by 1962 the worldwide production of HDPE had already reached 200,000 tonnes. Today the quantity of HDPE produced worldwide every year is estimated at 35 million tonnes (2007; all polymerising processes). By 2010, this figure is predicted to rise further to 40 million tonnes of which approximately 4 million tonnes will be processed into pipes.

The first HDPE pipes had to compete against traditional materials that had been in existence for many years and, therefore had a track record with the water supply companies who needed pipe systems with a minimum service life of 50 years. As Ulrich Schulte states in his inspirational work: A vision becomes true – 50 years of pipe made of HDPE: Creep tests on HDPE pipes were begun as early as the middle of 1954. Dr. Kurt Richard, the then Head of the Materials Department at Hoechst, adapted to plastic the existing method of predicting the service life of high-temperature steels. He applied the Larson-Miller correlation to the stress characteristics of HDPE. In 1959, after only 4 ½ years of testing at 80 °C, extrapolation of the time-to-failure values already permitted a prediction of the service life of the tested pipes to be 50 years at 20 °C. Two pipe specimens installed in October 1956 are still undergoing creep tests at a temperature of 20 °C and under hoop stresses of 5 and 7.5 N/mm² respectively. On 18th October 2006, the extrapolations of the pioneers of pipe life prediction were finally and most impressively confirmed! The findings obtained in this concrete case of application are of fundamental importance, as the very same extrapolation method is used for predicting the service life of todays generation of HDPE pipe materials.

This work states that the minimum service life of HDPE is 50 years. The success of the global use of HDPE pipes led to a significant improvement of the calibration and extrusion equipment but most importantly to the improvement of HDPE pipe materials. The first HDPE pipes installed in Germany were made from the so called first generation material. Nowadays we are using fourth generation materials which, has resulted in considerable improvement in the design life. The following graph demonstrates clearly that the design life of pipes manufactured from high performance materials is well above 100 years. It is also important to emphasise the considerable development to the additives, like new types of carbon black, as well as the new generation of antioxidants which provide further improvement to the final quality of the pipe. Of course we should not neglect the fact that first generation HDPE pipes installed over 40 years ago are still in use without any signs of fatigue. Ulrich Schulte refers to an example of some first generation pipes which were installed in 1955 at Hoechst AG, a chemical company in Frankfurt am Main, in 1955.

The pipe system had been used for the supply of fresh water to an entire floor of the building. It could be assumed that the pipes had provided a continuous flow of oxygen-saturated water for over 40 years. Some of the pipes were removed from the building and subjected to tests in order to establish the residual life expectancy. The results were impressive concluding to a value of at least 28 years. This clearly shows that the pipes would have exceeded by far the expected service life of 50 years.

There is a wide range of articles referring to tests that evaluate the design life of HDPE pipes. Dr. Lars-Eric Janson has published a large number of these articles referring to tests made in HDPE pipes. In his tests, one pipe was compressed to 5% vertical deflection between parallel plates and held, in some cases for nine years. The stiffness regression recorded, directly relates to the relaxation of the modulus of elasticity. In another series of tests, pipe was deflected to 4.3% and 13.6% and held for eight years, with the same regression being recorded on a log scale. The regression curves can be projected to 100 years, which is slightly over one order of magnitude from the period tested. Furthermore, Dr. Janson found that tests conducted for as little as 100 hours were sufficient to make safe extrapolations up to 50 years or more for PE pipe.

As years pass we will encounter examples showing how HDPE pipe stands up to the tests of time. Meanwhile designers and specifiers can enjoy peace of mind in knowing that rigourous scientific testing demonstrates clearly that this material has the staying power of several generations. However, in retrospect, if the Romans had been given access to this durable, flexible solution over 2000 years ago, would they have in fact, favoured its more traditional counterpart, concrete?