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INTRODUCTION

It was recognized early in the evolution of the thermoplastics pipe industry that in consequence of its viscoelastic nature, the fracture strength of a thermoplastic polymer is significantly influenced - for any given set of conditions of temperature and environment - by duration of loading. The longer a load is sustained; the lower is the fracture strength. It was also recognized that the long-term strength of a thermoplastics composition is not only determined by the primary ingredient, the base polymer, but can also be profoundly influenced by the nature and quantity of ingredients - such as property modifiers, processing aids, stabilizers, and colorants - that are used to enhance performance and facilitate processing, and to give product identification. But in those early days there existed no standard method by which reliable design stresses could be established for thermoplastic compositions intended for pressure pipe. All too often, design stresses were based on results of relatively short-term loading with safety factors that - based on experience, educated guess, limited experience, or other rationale - were said to adequately compensate for the reduction in long-term strength that characteristically occurs with all plastics when subjected to prolonged loading. Unfortunately, this approach was not only inconsistent from material to material, but oftentimes it was unreliable. The long-term strength of some materials was overestimated, while that of others was underestimated.

To remedy this situation, the Thermoplastics Pipe Division (subsequently named the Plastics Pipe Institute) of the Society of the Plastics Industry established in November 1958 the Working Stress Subcommittee, the predecessor to the Hydrostatic Stress Board (HSB), consisting of technical persons well versed in the state-of-the-art of the evaluation and forecast of the long-term strength of plastics. Two and half years later, in April 1961, this group agreed on a uniform Tentative Method for Estimating Long-Term Hydrostatic Strength and Hydrostatic Design Stress of Thermoplastic Pipe; and in July 1963, it issued its first hydrostatic design stress recommendations for compositions for which data had been submitted in accordance with this method.

A frequent challenge to the HSB was the evolutionary nature of the industry, particularly in the case of PVC pipe compositions. At first, each PVC composition was a fixed and very specific composition, with the use level and identity of each ingredient spelled out. The ingredient identification would often consist of a manufacturer's trade designation. In search of more effective and less costly sources of ingredients, companies holding listings for PVC compositions would often qualify alternative sources of a certain ingredient. To demonstrate qualification a company had to submit to the HSB extensive long-term data that showed the proposed change would not compromise a listed composition's longterm strength. From the knowledge learned by this work, policies were developed whereby a newly proposed ingredient, for example, a calcium carbonate - can qualify for that purpose provided it is demonstrated that its physical and chemical properties comply to requirements that have been established for that class of ingredient based on results of industry wide testing. These policies provided for prequalification of ingredients that greatly facilitated the process of determining equivalence.