The weathering of polymers is reviewed with attention concentrated on the mechanisms of degradation and stabilization, the methods of testing weatherability, and the predictive modelling of weathering behaviour. An introduction to the chemical mechanisms of degradation and stabilization is given and reference made to some of the many reviews available in the literature. Significant emphasis is placed here on engineering aspects, such as the way that weathering influences fracture mechanisms. The difficulties associated with relating accelerated laboratory tests with outdoor service behaviour are discussed. The complexities of the degradation processes limit modelling of the rate of degradation to rather specific systems and the predictions cannot be generalized.

Weathering of polymers: mechanisms of degradation and stabilization, testing strategies and modelling

Prediction of the remaining lifetime of polyethylene pipes after up to 30 years in use

The influence of fillers on the rheology of polymer melts is reviewed, together with an account of mechanisms involved during their combination in melt-mixing procedures. The application of these principles to the design and operation of industrial compounding technologies is then discussed. Means for inducing further microstructural changes during secondary melt processing are described, leading to the achievement of enhanced composite performance.

Rheology, Compounding and Processing of Filled Thermoplastics

Cyclic crack growth tests with CRB specimens for the evaluation of the long-term performance of PE pipe grades

A fracture mechanics concept for the accelerated characterization of creep crack growth in PE-HD pipe grades

Three different pipe-grade polyethylenes, in the form of one large and three small diameter pipe systems, have been tested at elevated temperatures, using constant and fluctuating internal pressure loadings that resulted in brittle fractures. The behaviour under fatigue of two of the three types of small diameter polyethylene pipes was substantially described by a cumulative damage model, whilst the third exhibited a fatigue weakness, an observation not previously reported. The performance of the large diameter pipes under fatigue was dominated by the presence of large voids in the pipe wall that arose from incorrect processing and resulted in premature failure. The sites of crack initiation in one material grade of the small diameter systems were examined in detail. In particular the size, position and composition of particles initiating fracture were determined. The maximum particle size on the fracture surface of the pipe was found to correlate reasonably well with a measure of pipe lifetime, as predicted by a fracture mechanics approach, and indicated that the lifetime of this one type of polyethylene pipe was dependent on the size of the inclusions initiating fracture.

The performance and causes of failure of polyethylene pipes subjected to constant and fluctuating internal pressure loadings

Processing conditions, microstructure and mechanical property correlations have been explored in injection-moulded plaques of an acetal copolymer. Barrel temperature was varied systematically between 453 and 503 K, with a constant mould temperature of 343 K. The microstructure and texture were determined by polarized light microscopy and X-ray pole-figure analysis, respectively. The overall structure of the mouldings was layered through the thickness and symetrical about the moulding centre line. At all barrel temperatures five layers were present: the outer three layers possessed significant preferred chain-axis orientation in the crystalline phase, while the two layers at the centre of the moulding were equiaxed. The texture and morphology of each layer is described and related to a model of mould filling. Increases in the barrel temperature reduced the extent of the outer oriented layers while increasing the extent of the equiaxed layers. Tensile tests were conducted on samples taken at 0° and 90° to the injection direction. Increases in barrel temperature had no influence on modulus but decreased both (α=0 and 90°) engineering yield stresses. The yield stress could be correlated with the extent of the oriented layers within mouldings. At all barrel temperatures the yield stress was greater when α=90°; this behaviour is explained in terms of a composite model.

The structure and mechanical properties of an injection-moulded acetal copolymer

At low temperatures and hoop stresses, polyethylene pipes fail by the time-dependent propagation of a crack. These brittle, fissure-like failures have been observed to initiate from adventitious flaws, and the concepts and methods of fracture mechanics indicate that flaw size should determine stress rupture lifetime. A number of controlled model experiments have therefore been undertaken to assess the influence of flaw size and material toughness on the stress rupture lifetimes of polyethylene pipes. To two different pipe grade polyethylene resins (one shorter, one longer lifetime resin) flaws of varying sizes have been added. For the shorter lifetime resin small flaws were, in addition, purposely excluded by the use of fine melt filtration techniques. Pipes containing added flaws or pipes where flaws were excluded were then stress rupture tested under those conditions designed to induce brittle failure by slow crack growth. The stress rupture lifetimes of the various pipes are then correlated with flaw size. The results of the tests using the shorter lifetime resin show that flaw size does have a significant influence. It is particularly interesting to note that melt filtration, which removes large inherent flaws, substantially improved the stress rupture lifetime. With respect to material toughness, the longer lifetime pipe grade polyethylene resin showed a healthy tolerance to included flaws. In respect of the stress rupture test preferred resins can therefore be identified in terms of their tolerance to included flaws.

An examination of the role of flaw size and material toughness in the brittle fracture of polyethylene pipes

Fatigue properties of high density polyethylene pipe systems

The long-term behaviour of an aluminium-reinforced polyethylene pressure pipe has been explored by undertaking stress rupture tests at 60 and 80 °C. The results of the tests showed these macrocomposite pipes have a time-dependent strength, such that with an increasing time under load the strength declined. In addition the pipes were weaker at 80 °C when compared to the 60 °C strength. The analysis of the influence of time and temperature on strength showed these multilayer pipes can be considered to behave as do conventional homogeneously structured plastic pipes, and that to describe the influence of time on the pipe strength, the accepted procedures developed for conventional plastics pipes can be applied. In addition the mode of failure of the pipes was examined. Pipe failure initiates by the strain-controlled failure of the reinforcing aluminium layer. The polyethylene layers then fail almost instantaneously in a ductile mode. This analysis of the mode of failure was supported by freeze-thaw cycling tests to − 25 °C and the 60 and 80 °C stress rupture tests.

J. Bowman

Papers

The performance and causes of failure of polyethylene pipes subjected to constant and fluctuating internal pressure loadings

The structure and mechanical properties of an injection-moulded acetal copolymer

An examination of the role of flaw size and material toughness in the brittle fracture of polyethylene pipes

Fatigue properties of high density polyethylene pipe systems

The long-term behaviour of an aluminium-reinforced polyethylene pressure pipe