Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
1996-06-24
2001-05-01
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C428S377000, C428S461000, C428S516000, C138SDIG006, C138S145000, C424S412000
Reexamination Certificate
active
06224957
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved anti-corrosive material. More particularly, the present invention relates to a multi-layered material used to encase buried conduits to prevent corrosion.
BACKGROUND OF THE INVENTION
underground conduits are ubiquitous in modern society. Conduits are used for carrying a wide variety of substances, including water, natural gas, oil, and sewage. In the past, these conduits have been typically comprised of plastics, concrete, or metals. One of the major problems with buried conduits comprised of concrete, concrete with metal reinforcements, or metal, is corrosion. The severity and rate of corrosion is dependent on the type of material comprising the conduit and the environment in which the conduit is buried. For example, ductile iron pipe (DIP) typically exhibits a low risk to severe corrosion compared to other metals. However, a rapid increase in the corrosion rate can be initiated by oxygenated water, tidal action, or specific soil types as soils containing sulfides. Because of high costs associated with removal and replacement of corroded conduits, the industry has expended substantial resources to solve this problem.
Initially, conduits were covered with paint coatings, wraps, or other materials to separate the conduit surfaces from the environment. Later, barrier films of polyethylene were used to protect DIP conduits. By insulating the exposed surfaces from soil, electrical currents, and oxygenated water, corrosion is usually prevented. However, due to improper installation, tears and punctures to the barrier film occurring during the installation and backfill process, free flow of water from tidal action, or soil or water becoming entrapped between the film and the conduit surface, actual corrosion still occurs in many cases. The industry has attempted to solve these problems by using more durable barrier films to encase the conduit surfaces, such as high density cross-laminated polyethylenes (HDCLPE). The superior impact strength, tear resistance, and tensile strength of HDCLPE has reduced some of the problems associated with the installation and backfill process; however, HDCLPE does not adequately address or control the problem of corrosion. Since there has not been an adequate alternative, present industry standards typically use either an 8 mil low density polyethylene (LDPE) film or a 4 mil HDCLPE film, a mil being equal to one thousandth of an inch (0.0254 millimeter), to wrap around the conduits for protection against corrosion.
Polyethylenes, as well as other plastic films, limit the free flow of water against the conduit surfaces, thereby reducing available oxygen. Any moisture which becomes trapped between the film and the conduit surface will eventually become deaerated. A problem arises where deaerated water levels are attained in the presence of sulfate reducing bacteria. Many anaerobic bacteria, such as
Desulfovibrio desulfunicans
, thrive in certain fresh water, brackish water, sea water, sulfate soils, or warm soil conditions. These bacteria act as a catalyst to initiate or augment the rate of corrosion in an environment that is normally adverse to corrosion. Additionally, other types of bacteria are believed to play a part in corrosion propagation and it appears that bacteria are also responsible for degradation of the polyethylene film. A possible solution to this problem is to treat the materials used to encase the conduit with bactericides. However, most bactericides are topical and water soluble, thereby offering only initial protection. Since conduits are buried for decades, this would not provide adequate long-term protection.
Another possible solution is to use certain volatile corrosion inhibitors (VCIs) which can be added to eliminate or reduce the presence of corrosion. An example of a commonly used VCI is illustrated in U.S. Pat. No. 3,425,954. These VCIs can be used to prevent conditions from developing inside the film barrier which are favorable to corrosion. VCIs work at a micron level to passivate the surface of metal with a passive film, thus reducing the chemical reactivity of its surface. VCIs are normally used in kraft papers for short term protection of metal parts, as illustrated in U.S. Pat. No. 4,557,966; however, paper is not suitable to be buried. VCIs could be added to the polyethylene film, but the effectiveness will be shortened since the vapor tends to escape from the film, thus preventing extended protection.
From the foregoing it may be seen that a need exists for an improved anti-corrosive material for protecting conduits buried in conditions favorable to corrosion.
SUMMARY OF THE PRESENT INVENTION
It is the object of the present invention to provide an improved anti-corrosive material which is superior to those presently used to protect buried conduits.
It is another object of the present invention to provide a material which contains a biocide to prevent bacterial-induced or enhanced corrosion.
It is still another object of the present invention to provide a material which contains a biocide to prevent bacterial-induced or enhanced degradation of the material.
It is a further object of the present invention to provide a material which contains volatile corrosion inhibitors to prevent corrosion.
It is still a further object of the present invention to provide a material which is co-extruded, calendered, or laminated into a multi-layered material comprising a conduit contacting layer having any anti-corrosive agents, such as biocides or volatile corrosion inhibitors, impregnated therein such that the anti-corrosive agents can migrate within the conduit contacting layer to contact the conduit surface and prevent corrosion.
It is yet a further object of the present invention to provide a material which is co-extruded, calendered, or laminated into a multi-layered material further comprising a barrier layer adjacent the conduit contacting layer which prevents the anti-corrosive agents from penetrating the barrier layer and escaping into the surrounding environment.
These and other objects of the present invention are accomplished through the use of an improved anti-corrosive material used to protect buried conduits from corrosion. The material is a multi-layered co-extruded, calendered, or laminated polyolefin. The material has an outer layer, or environment contacting layer, preferably comprised of a low density polyethylene having characteristically strong tensile strength and elongation properties to provide conventional protection from soil, water, air, or other potentially damaging elements. The material has a center layer preferably comprised of a high density polyethylene having superior tensile strength to provide a high density barrier between the outer layer and an inner layer. The inner layer, or conduit contacting layer, is preferably comprised of a low density polyethylene impregnated with a biocide, a volatile corrosion inhibitor (VCI), or both. The biocide and VCI are able to migrate through the low density polymer matrix but the rate of migration of the biocide is considerably slowed through the high density polymer and the VCI is essentially unable to penetrate it. Thus, the biocide and VCI are substantially prevented from escaping into the surrounding environment, but rather, are trapped within a “protection zone” immediately adjacent the conduit surface to provide extended protection against corrosion.
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patent: 42
Crook John A.
Wilson, Jr. John H.
Burr & Forman LLP
Fulton Enterprises, Inc.
Jackson Robert M.
Tarazano D. Lawrence
Thibodeau Paul
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