Abstract:
A plastic pipe with a hard outer shell lined with an inner shell of soft resilient plastic foam with a smooth central axial passageway adapted to be a conduit for liquid. A method of manufacturing this pipe which comprises simultaneously extruding the material for the outer shell and the foamable material for the inner shell into a pipe forming die.

Description:
BACKGROUND OF THE INVENTION 
     Everyone who has lived in a house located in a climatic zone which includes periods of freezing weather has had to be concerned about the possibility of water pipes freezing and bursting. Measures commonly used to prevent the freezing of water pipes include insulation of all pipes, running water continuously through the exposed pipe, and heating the pipe. None of these measures is entirely satisfactory and it has long been an engineering hope to provide a simple inexpensive pipe that would, in temperate climate zones, be able to withstand temperatures down to about 0° F. without permitting the pipe to burst due to freezing of the water inside. Since ice has a volume of about 110% of the water required to produce that ice, any freeze resistant pipe must either be strong enough to prevent the volume expansion or the pipe must be sufficiently expandable to accommodate the expansion. The availability of resilient plastic materials has provided hope of making a freeze resistant pipe, but until this invention such a pipe could not be made economically or commercially acceptable. 
     It is an object of this invention to provide an inexpensive freeze resistant plastic pipe. It is another object to provide a practical method for preparing such a pipe. Still other objects will become apparent from the more detailed description which follows. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention relates to a pipe having a smooth rigid hard outer shell, a smooth flexible inner shell spaced inwardly from said outer shell, and a flexible closed-cell foam material filling the space between said outer and inner shells. 
     This invention also relates to a process for prepring a pipe having a smooth hard outer shell spaced apart from a smooth flexible inner shell separated by a foam material, said process comprising extruding a first tubular structure of polyvinyl chloride and simultaneously extruding inside of said first tubular structure, and in contact therewith, a second tubular structure of a foamed polyolefin having a smooth continuous interior cylindrical surface. 
     In specific embodiments of this invention the outer shell is polyvinyl chloride and the inner shell is foamed polyethylene, and the pipe is made by simultaneously coextruding foamed polyethylene tube inside an outer tube of polyvinyl chloride. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a longitudinal cross section of a coextrusion apparatus for forming the pipe of this invention as taken at 1--1 of FIG. 3; 
     FIG. 2 is an end view of the pipe of this invention; and 
     FIG. 3 is an end elevation of the coextrusion apparatus of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The features of this invention can best be understood by reference to the attached drawings. In FIGS. 1 and 3 there is illustrated a coextrusion apparatus for preparing the pipe of FIG. 2. The die 9 of this apparatus is a plurality of concentric annular spaces and forming rings into which two types of molten plastic materials are caused to flow and subsequently be congealed into the finished pipe product. The outer shell 10 of the pipe is formed by pumping molten plastic from extruder exit 14 into annular space 24 between outer ring 15 and inner ring 23 and flowing to the right as indicated by arrow 25. Preferably, exit 14 approaches annular space 24 tangentially so as to provide the easiest of flow conditions for the molten plastic to fill space 24. At the same time inner shell 11 is formed by pumping a second molten plastic from a second extruder exit 16 into the annular space 18 between inner ring 23 and core 19 in the direction of arrow 28. Shells 10 and 11 are caused to move to the right at exactly the same speed and to be welded together along interface 29 to produce a unitary product. The outside of the composite pipe passes through sizing collar 30 to smooth the outside surface and to give it a precise outside diameter. The composite pipe moves to the right of sizing collar 30 into a cooling zone, e.g., a pool of water, to bring the temperatures of the plastic materials below the softening temperature to a level where the pipe will not change in size. 
     Preferably, the material of outer shell 10 is a strong hard material, such as polyvinyl chloride, ABS copolymer, or the like. The material of inner shell 11 is a soft foamable material such as foamed polyolefin, e.g., polyethylene. When the extruded foamable polyolefin passes beyond die exit 27, space 18 becomes larger permitting the foaming action to take place and allowing inner shell 11 to contact outer shell 10 along interface 29. The plastic material in both outer shell 10 and inner shell 11 is still somewhat molten at this point and the two materials, if compatible, will tightly adhere to each other along interface 29 to form a tight weld as the materials cool and solidify. 
     Core 19 is preferably a forming surface for the inside of the pipe, and also a cooling component. Water or other coolant is introduced into central pipe 20 and flows countercurrent to the movement of inner shell 11 in annular space 26 and exits at 21. Core 19 serves to cool the molten plastic in space 18 so that the inside surface 13 of the pipe will be sufficiently solidified to hold its shape while the outer surface of inner shell can expand through its foaming capability after passing die exit 27. 
     The product pipe is shown in FIG. 2 as having a hard outer shell 10 bonded at interface 29 to inner shell 11 at a zone of foamed plastic 12 while the inside surface 13 is smooth continuous surface having substantially no surface irregularities that would adversely affect fluid flow in the pipe. When such a pipe is filled with water and subjected to freezing conditions the water will eventually form ice causing the volume to expand about 10%. Under these conditions inner shell is expandable due to the foamed zone 12 where the foam cells are compressible. Inner surface 13 is elastic sufficiently to permit such expansion. Since the volume varies by the square of the diameter, it is apparent that a very small expansion of the diameter will cause a large change in volume. 
     The foam capability of the molten plastic in extruder exit 16 is accomplished by known means of incorporating a blowing agent in the plastic. Such materials are those that release a gas at elevated temperatures, such materials as azo compounds, ammonium compounds, carbonate compounds, etc. Preferably the flowing agent is a solid which is incorporated into the molten plastic at 16 and dispersed intimately throughout by means of a suitable mixing means 17. When the pressure on the molten plastic is released as at die exit 27, the blowing agent gasifies and produces foam cells along interface 29. 
     While the invention has been described with respect to certain specific embodiments, it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the spirit of the invention. It is intended, therefore, by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.