scope:

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 long-term 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.

Later on, during the seventies, the concept of "PVC range compositions" was introduced. As new and improved extrusion technology began to be used, it was discovered that fine-tuning the quantity of certain ingredients - particularly, so called "internal" and "external" lubricants - would greatly benefit production rate and product quality. To avoid the impracticality of having to qualify each composition variation, industry expanded their fixed content recipes into range compositions, whereby the allowable content of certain ingredients was defined not by a fixed amount but rather, by a minimum/maximum range. The acceptable ranges of ingredient content had also to be established by long-term data documentation.

To make it possible to use resins and other ingredients from different suppliers, extruders of PVC pipe had to qualify a number of "different" range PVC compositions - each provided by a different resin supplier - which in fact were often times quite the same except for source of resin, or some other key ingredient. For a manufacturer of a new ingredient, or of an alternative to an existing ingredient, having to qualify his product for inclusion in the many private PVC range compositions that then abounded was a costly and time consuming process. In recognition of this situation, the HSB proposed in 1983 to establish a single, generic and public PVC range composition which then included all PVC resins and ingredients which had been qualified in privately held compositions of the same kind. To accomplish this required the cooperation of all major holders of PVC compositions, including their willingness to share with the HSB their confidential recipes. This cooperation was obtained, the compositions of the many stress-rated compositions were compared, and a single, generic, state-of-the-art composition was established which allowed a wide choice in PVC resins and in ingredients.

The HSB next worked on defining the policies and procedures for qualifying new ingredients. In 1985 the PPI PVC range composition and related policies were agreed upon and are published in this report. Since that time many new alternate ingredients have been qualified for use in this composition.

The policies and procedures in this Technical Report are intended to cover ingredient listings for most PVC piping applications. PPI recognizes there may be unusual cases, issues or circumstances that are not covered in TR-2, and that may justify an exception to the standard policies. To allow manufacturers an opportunity to have their ingredient(s) listed by PPI when this occurs, the HSB has provided a "Special Case" system. The manufacturer may present its "Case" to the HSB at one of their two annual meetings, usually in February and August, using the approved "Special Case" form in TR-3. All information provided to HSB in these special cases will be made available for review only by HSB members and PPI staff, and will be held by them in strict confidence, in accordance with PPI's written confidentiality procedures (available from the HSB Chairman). There is a PPI fee for each special case. You must contact the HSB Chairman well in advance of each meeting to arrange for your special case. A completed HSB submission form must be received at least two (2) weeks prior to the HSB meeting to permit HSB consideration at that meeting.