Pipes and tubular conduits – Distinct layers – Bonded to each other
Reexamination Certificate
2001-03-28
2003-08-12
Brinson, Patrick (Department: 3752)
Pipes and tubular conduits
Distinct layers
Bonded to each other
C138S141000, C138SDIG008, C138S140000
Reexamination Certificate
active
06604552
ABSTRACT:
This invention relates to a metal-plastic multilayer pipe with a plastic inner pipe and metal sheath around the plastic inner pipe, suitable for use in hydronic heating systems and for domestic hot and cold water plumbing systems.
BACKGROUND OF THE INVENTION
Plastic Tubing For Hydronic Heating and Plumbing
In hydronic heating systems good quality “cross-linked” polyethylene tubing can carry water at temperature in excess of 140° F. without any deterioration of the tubing or the tubing oxygen barrier. The design criteria of plastic tubing for radiant floor and wall hydronic heating systems is determined by a number of important factors to insure an absolutely safe and reliable tubing. The same design criteria are often demanded for hot and cold water domestic plumbing systems. The most important design criteria requirements are:
1. High resistance to temperature aging for water temperatures up to 200° F.
2. High resistance to stress cracking.
3. High resistance to chemical solvents (water additives, antifreeze solutions, concrete additives).
4. Lowest possible linear thermal expansion.
5. High tensile strength.
6. High form stability.
7. High resistance to abrasion.
8. High resistance to deformation.
9. Dimensional tube tolerances.
9. Internal and external tube wall smoothness.
10. Behavior during long term internal pressure creep test which takes into account the temperature-dependent aging behavior of the pipe material at water temperatures up to 200° F.
Many of these requirements are dictated by the usual practice of embedding the tubing in a layer of concrete. They are design criteria that are outlined and specified in the ASTM standards (American Society for Testing and Materials), and DIN (German Industry Standards). Many, if not all of these design requirements can be achieved while still retaining a flexible and workable plastic tubing (pipe) as an end product. That tubing is called PEX, which is short for “Polyethelene Cross-Linked”. PEX has been synonymous with plastic heating pipe in many European countries for several decades and has a track record that has made it the plastic tubing of choice for hydronic heating applications. Long term bench tests, which simulate 30 years of continuous use, in addition to accelerated testing which projects pipe performance well in excess of 30 years has confirmed the excellent long term real service life track record of PEX. Crossed-linked polyethylene tubing is now, after 20 years of use and improvements, the most widely accepted pipe material in the European plumbing industry for both heating and plumbing applications.
Plastic Tubing and Cross-Linking
The molecules of any plastic material tend to slip and slide over one another fairly freely. As ambient and water temperatures rise, the plastic material softens and finally melts. This thermal oxidation of plastic material is a long term aging process which will eventually result in pipe failure.
To combat this premature aging the molecules within the tubing are realigned in order to give greater stability to the material itself. The cross linking process takes place within the molecular structure of the plastic material. The most common thermoplastic materials currently being used for heating and plumbing pipe, often referred to as polyolefin materials are: Polyethylene (PE); Polypropylene (PP); and Polybuten (PB)(generic term for polybutylene). Among this family of polyolefin plastics, only Polyethylene has been determined to have the molecular structure which lends itself perfectly to the cross linking process.
“Un-cross-linked” polyethylene (PE) tubing, as it leaves the extruder where it receives its basic pipe dimension and wall thickness, is composed of long hydrocarbon string molecules forming a loosely held together array of hydrogen and carbon atoms which can be compared to a beaded curtain swaying in a breeze. This is basically the molecular composition of the PE tubing which is available at hardware stores and is suitable only for non-critical applications. A material, in this form, is not suitable for heating and plumbing applications. Within a relatively short period of time the pipe material fatigues under the stress of water temperature and pressure as well as temperature cycling and the beaded curtain of molecules splits open without resistance. By cross-linking the molecular “beads” (hydrocarbon string molecules), forming cross-connections which are referred to as cross-linking bridges, the string molecules form a three dimensional network of hydrocarbon molecules. The “beaded curtain” becomes transformed into a fishing net with strength and stability.
In this way, the previously non-applicable PE pipe is transformed, after cross-linking, into a completely different material with all the desired characteristics demanded for a heating or plumbing pipe. After the crosslinking of the PE tubing, its molecular mobility is severely impeded by the cross-linking bridges between the string molecules. The material does not flow or melt and its form becomes stable against heat. The material holds its shape at all temperatures, even exposed to blow torch temperatures until it chars or burns. The thermoplastic has been transformed into a thermoset material by cross-linking, eliminating the melting point or liquid phase of the material. The PE pipe has been transformed into PEX pipe
Cross Linking Techniques
There are basically two types of PE raw materials in use: Low to Medium density (LD or MD PE) and High density (HD PE). Low to Medium density polyethylene “SOFT PE” has a multibranch string molecule shape which allows a lower to medium density formation of string molecules within the pipe PE material.
High density polyethylene (HD PE) has a linear string molecule shape with only small stumps of branches, which allows for a higher density formation of string molecules within the pipe PE material.
The material density affects the physical properties of the pipe material. HD PE or “HARD PE” has a higher resistance to stress cracking and chemical solvents, higher tensile strength, higher resistance to deformation and is less permeable to oxygen
Chemical Cross-linking techniques include: Peroxide Cross-linking; Silan Cross-linking via Dow Corning Method; and AZO Cross-Linking. Three methods of Peroxide Cross-Linking are the Engel, PAM and DAOPLAST methods.
Mechanical Cross-linking is Electronic Cross-linking by a Cross-Linking High Energy Electron Beam
The various chemical cross-linking methods use chemical agents which are added to the PE base resin in order to form cross-linking bridges between the PE string molecules. The only practiced mechanical cross-linking method uses no chemical agents, instead, utilizes the high energy of an electron beam accelerator to form a three dimensional cross-linking network between the PE molecules.
Among the various chemical methods only two types are commonly used for heating pipe production: The Engel and Silan method. The Engel method, named after its inventor, uses a cross linking agent (peroxide) and heat stabilization agents which are mixed into the PE resin. The mixture is then compressed under high pressure in a “pre-molten” state and fed through the extrusion die, where the actual cross linking process takes place. This is a “press-sintering” process which achieves pipe extrusion and molecular cross-linking during one extrusion process.
The Silan method uses a mixture of two compounds with a mixing ratio of 95 to 5 parts. One compound consists of PE resin and cross-linking agents as well as other additives. The second compound consists of PE resin and a catalyst. After mixing both compounds, the pipe is extruded conventionally. The cross-linking reaction is triggered after extrusion by exposing the extruded coil to moisture such as steam or water. Most other chemical methods are variations of either the Engel or Silan method.
The electronic or mechanical cross linking method does not use any chemical means to achieve cross-linking bridges between the PE molecules. The basic PE resin is first extruded to give the pipe the basic requir
Feldkamp Alfred
Freermann Reinhold
Hansen Jorg
Riesselmann Franz-Josef
Brinson Patrick
Dunn, Esq. Robert T.
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