GB2206545A - Drag reduction composite - Google Patents

Abstract

A drag reduction composite for attachment to solid surfaces to provide for drag reduction of liquids flowing over such surfaces is disclosed. The composite comprises an active outer layer and means in the form of an inner layer for attaching the outer layer to a solid surface. The active outer layer comprises a finely divided drag reducing polymeric material which is both liquid hydrophilic and liquid soluble blended in a liquid insoluble polymeric material to form a heterogeneous mixture of the drag reducing polymeric material dispersed throughout the insoluble polymeric material. The insoluble polymeric material reinforces the outer layer to provide a pliable integral outer layer.

Description

DRAG REDUCING FILM FOR APPLICATION TO VESSEL HULLS FIELD OF THE INVENTION This invention relates to a drag reducing film which is suitable for application to solid surfaces so as to provide drag reduction while liquids flow over the surfaces.

BACKGROUND OF THE INVENTION There are many situations which require the use of drag reducing materials to provide an energy efficient system. For example, drag reducing materials have been used on boat hulls to minimize turbulence along the boat hull surface and hence gain increased speed.

Traditional methods of drag reduction may be typified by the following methods: 1. Application of a smooth coating of a high gloss finish to the boat hull. The coating is then bluffed to produce a surface with minimum defects and hence induce minimal turbulence.

2. Pour suitably selected materials at the front of the travelling boat or vessel. Such materials are selected to reduce turbulence along the water/boat surface interface and thereby reduce drag.

3. Coat the vessel hull with a selected hydrophilic coating composition.

An effective hydrophilic coating consists of polyethylene oxide in water which is applied to the boat hull surface. This coating reduces drag, however is of minimal commercial value because the polyethylene oxide is removed from the boat hull surface in a very short interval, hence drag reduction is achieved for a minimal period of time.

Applicant has described in copending U.K. patent application filed April 25, 1988, Serial Number 8809751.4 a drag reducing coating for application to boat hulls and the like. The drag reducing coating has the ability to slowly release a drag reducing agent at the surface to which the coating is applied. The coating provides for prolonged drag reducing characteristics as the surface carrying the coating has liquid passing thereover. The coating composition comprises a drag reducing polymeric material in a paint base composition or other suitable carrier. The carrier may optionally include fillers. The coating composition, when applied, dries to a smooth finish.

The active drag reducing agent is slowly released to the outer surface of the dried coating to provide a continuous form of drag reducing agent along the surface of a structure. This type of coating is suitable for short term functions, such as half-day boat races, and some military applications, such as under water torpedoes. There are, however, applications where the drag reducing agent has to be effective over extended periods of time. This would entail application of a very thick coat of carrier which is difficult to achieve.

SUMMARY OF THE INVENTION According to this invention, a thick coating has been prepared which carries the active drag reducing agent and releases same over extended periods and at the same time provides for attachment of the thick coating to the desired solid surface.

According to an aspect of this invention, drag reducing coating thicknesses considerably in excess of paint coat thicknesses is provided. A drag reduction composite for attachment to solid surfaces provides for drag reduction of liquids flowing over such surfaces.

The composite comprises an active outer layer and means in the form of an inner layer for attaching the outer layer to a solid surface. The active outer layer comprises a finely divided drag reducing polymeric material which is both liquid hydrophilic and liquid soluble blended in a liquid insoluble polymeric material to form a heterogeneous mixture of the drag reducing polymeric material dispersed throughout the insoluble polymeric material, the insoluble polymeric material reinforcing the outer layer to provide a pliable integral outer layer.

According to another aspect of the invention, a method for reducing drag at a liquid/solid interface as liquid flows over a solid surface by affixing a composite which includes a drag reducing outer layer to the solid surface. The outer layer comprises an outer active layer of a finely divided drag reducing polymeric material which is both liquid hydrophilic and liquid soluble blended in a liquid insoluble polymeric material to form a heterogeneous mixture of the drag reducing polymeric material dispersed throughout the insoluble polymeric material. The insoluble polymeric material reinforces the outer layer to provide a pliable integral outer layer. The outer layer is affixed to the solid surface by use of a liquid insoluble pressure sensitive adhesive.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The active outer layer of the composite may be provided in sheet form. The sheet may be developed by either extruding or pressing the materials of the active layer into a suitable size sheet. The developed sheet is attached to the working surface by a suitable attachment medium. For example, the sheet may be provided on one side with a pressure sensitive adhesive protected by a peelable film. The film may be removed and the sheet directly attached to the working surface by applying pressure to the sheet to secure the adhesive to the solid working surface.

Alternatively, according to another embodiment of the invention, the active layer of the composite may be either laminated or coextruded with the substrate layer.

The substrate layer carries on its underside a suitable adhesive for either permanently or temporarily securing the substrate to the desired solid surface. Optionally the adhesive may be of the pressure sensitive type which is water resistant and secures the substrate to the solid surface. When the active coating has been depleted of the active ingredients, the substrate with remaining coating may be removed from the solid surface to permit reapplication of a new layer of drag reducing material.

This type of drag reducing material having normally a thicker coating, such as in the range of 5 to 25 mils, has a variety of uses. The preferred range of thickness is 15 mils to 25 mils. Particularly preferred uses are on boat hulls and other surfaces which pass through water. The active drag reducing ingredients permit increased hull speeds with the same energy requirements.

Furthermore, the drag reducing agents reduce turbulence at the surface of the solid substrate. This has particularly important applications in the military where reduction in turbulence reduces audio noise at the hull surface. This minimizes the risk of detecting vessel travel by sonar or the like by other ships which is a significant advantage in war zones. The composition of this invention is equally applicable to underwater vessels like submarines and long range under water munitions, such as torpedoes, which now have a greater chance of going undetected during the extended life of the coating. In addition, reduction in background noise by application of such coatings to underwater bodies containing sonar detection equipment results in improved detectability of other vessels. The composition is particularly suited for use in saline solutions, lake water and sea water.

The active ingredient which provides for the drag reducing function is of the type which is slowly released into the adjacent liquid. Such drag reducing agents include guar which is a complex polysaccharide derived from plants. Such guars have friction reducing properties. Other known friction reducing compositions include poly(ethyleneoxide), polyacrylamides, sodium carboxymethyl cellulose, flax meal, hydroxyethyl cellulose, cetyltrimethyl ammonium bromide-1-naphthol and sodium polystyrene sulfonate. One or more of these drag reducing polymers may be added to the carrier material and laminated or coextruded with the suitable substrate.

A variety of poly(ethylene oxide) polymers are available which preferably are of a high molecular weight. According to an aspect of the invention the poly(ethylene oxide) resins sold by Union Carbide Corporation under the trade mark POLYOX have molecular weight in the range of 5 million. The formula for the POLYOX water soluble resins is (CH2-CH2-0)n which is part of the polyether chemical family. The preferred polymer has a melting point of 65"C with a bulk density of 384 kg3. Normally the polymer is a white powder which may be ground to a fine constituency, particularly if ground when frozen. It is appreciated that cryogenic grinding is very useful under these conditions.An impact mill is suitable for these applications where the working components are super cooled by use of liquid nitrogen which may be added to the mill along with the frozen feed of polymer particles. As the recovered ground polymer is reheated to room temperature, a desiccant may be used to ensure that moisture in the air does not condense on the cold polymer fines.

Various polyacrylamides are available for use as drag reducing agents such as SEPARAN AP-30 (trade mark) sold by Dow Chemical, K-PAM (trade mark) sold by American Cyanamid, J-100 (trade mark) sold by Dow Chemical, ET-597 (trade mark) sold by Dow Chemical, and POLYHULL-27 (trade mark) sold by Stein-Hall.

There are many sources of sodium carboxymethylcellulose such as CMC-7 HSCP (trade mark) available from Hercules Powder Company.

The drag reducing agent is normally incorporated into the carrier immiscible polymeric material in a finely ground form. However, it is appreciated that the drag reducing agent may be incorporated into the immiscible polymeric carrier material as globular components which are presented at the surface of the coating so as to slowly diffuse into the adjacent liquid.

The immiscible polymeric material used as a carrier for the drag reducing agent is of any suitable water insoluble material (i.e. immiscible in water) which is compatible with the drag reducing agent in not causing any degradation thereof. An additional advantage in using a polymeric carrier is that a suitable polymeric material may be chosen which provides reinforcing for the sheet to render it pliable yet integral layer.

Furthermore, the immiscible polymeric material is preferably of a composition which can be laminated or in other way bonded to the substrate material, such as by coextrusion of the layers. It is appreciated that the active layer may include an immiscible polymeric material which is adhered to the substrate by a suitable adhesive in either the lamination or co-extrusion process. A preferred form of immiscible polymeric material is chlorinated polyethylene. The active drag reducing material can be finally dispersed or, depending upon the drag reducing agent, dissolved in the water immiscible chlorinated polyethylene. Although it is appreciated that a variety of polymeric materials are available for this us, other representative materials include polyolefins, natural and synthetic rubbers, polyhalocarbons, polyacrylates and the like.A preferred form of drag reducing agent is polyethylene oxide. This material can be finely dispersed in the chlorinated polyethylene or in some situations, may be partially dissolved therein.

The substrate for the composite is a structurally stable water immiscible polymeric material. The substrate is of durable material which can carry a suitable adhesive for securing the substrate to the desired solid surface. There are a variety of pressure sensitive adhesives which may be used in combination with a compatible substrate to achieve the attaching of the substrate to the solid surface. A preferred substrate is chlorinated polyethylene which is fully compatible with the carrier of chlorinated polyethylene used in the drag reducing film.

The pressure sensitive adhesives may be of the type which can be applied under water and produce sufficient adherence to maintain the sheet in place on the solid surface during use. A preferred type of pressure sensitive adhesive is that marketed by 3M under the product number 9472-95B.

According to an aspect of the invention, the greater film thickness is achieved by using a carrier such as the chlorinated polyethylene in which the polyethylene oxide drag reducing agent is blended. The polyethylene oxide may be in finely ground form as dispersed and blended into the chlorinated polyethylene.

A film of desired thickness is made from this compounded material. The film may be prepared by extrusion or other arrangements involving pressure to make the film of a desired thickness and of reasonably consistent thickness. The produced film of the composite containing the active drag reducing agent may be then laminated to the substrate which, according to this preferred embodiment, is of the chlorinated polyethylene. By pressure and heat, lamination is obtained although it is appreciated that where dissimilarities occur between the substrate and the immiscible carrier polymeric material suitable adhesives may be applied in securing the outer layer to the substrate. Hence the substrate forms an attachment surface forth compounded film which is too weak and slippery to be directly secured to a solid surface, such as a boat hull.By this method, thick layers of the compounded material including the active drag reducing agent are possible. The leaching of the polyethylene oxide leaves porosity in the carrier material that permits further leaching at the desired rate.

Example 1- Method for Attachment of a Drag Reducing Film to a Vessel Hull Polyethylene oxide, such as Polyox (trade mark of Union Carbide) and chlorinated polyethylene- (Tyrin 2552 trade mark Dow Chemical) were mixed in equal weight proportions in a Brabender (trade mark) mixing head at 1600C for 20 minutes. The blended lump form was pressed between sheets of Mylar (trademark) release film for several minutes at 109"C and 10 tonnes pressure to make the film of approximately 0.015 inch thickness (15 mils). A sheet of chlorinated polyethylene was similarly prepared to a thickness of approximately 0.015 inch thickness. The blended sheet was then placed over the substrate sheet and repressed at similar conditions to laminate the sheets together by application of pressure and heat. A pressure sensitive adhesive was applied to the exposed face of the substrate. The substrate was adhered to an aluminum surface to simulate a bcat hull. With the polyethylene oxide/chlorinated polyethylene compounded outer layer immersed in water, slow release of the polyethylene oxide took place. The rate of release was a function of the concentration of polyethylene oxide in the compounded film to provide suitable drag reducing properties for use on vessel hulls and the like.

As an alternative to the lamination process defined in this Example, where the carrier for the drag reducing agent is similar to or the same as the substrate, it is appreciated that the material may be formed by coextrusion. The compounded carrier and drag reducing agent is coextruded as one layer in conjunction with an outer layer constituting the substrate. The pressure sensitive adhesive may be either coextruded to the back of the substrate or subsequently applied to provide a completed sheet material for application to the vessel hull surface.

Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.

Claims (9)

CLAIMS:

1. A drag reduction composite for attachment to solid surfaces to provide for drag reduction of liquids flowing over such surfaces, said composite being characterized by an active outer layer and means in the form of an inner layer for attaching said outer layer to a solid surface, said active outer layer comprising a finely divided drag reducing polymeric material which is both liquid hydrophilic and liquid soluble blended in a liquid insoluble polymeric material to form a heterogeneous mixture of said drag reducing polymeric material dispersed throughout said insoluble polymeric material, said insoluble polymeric material reinforcing said outer layer to provide a pliable integral outer layer.

2. A drag reducing composite of claim 1, wherein said attachment means comprises a substrate layer joined to said outer active layer, said substrate layer is a liquid insoluble material and has on its surface means for affixing said substrate layer to a solid surface.

3. A drag reducing composite of claim 1 or 2, wherein said drag reducing polymeric material is selected from the group consisting of guars, poly(ethylene oxide), polyacrylamides, sodium carboxymethylcellulose, flax meal, hydroxyethylcellulose, cetyltrimethylammonium bromide-1-naphthol and sodium polystyrene sulfonate.

4. A drag reducing composite of claim 1, 2 or 3 is adapted for use in saline solutions, sea water or lake water.

5. A drag reducing composite of any one of the preceeding claims wherein said outer layer in the form of a sheet and has a thickness in the range of 5 mils and up.

6. A method for reducing drag at a liquid/solid interfaces liquid flows over a solid surface by affixing a composite which includes a drag reducing outer layer to said solid surface, said outer layer comprising an outer active layer of a finely divided drag reducing polymeric material which is both liquid hydrophilic and liquid soluble blended in a liquid insoluble polymeric material to form a heterogenous mixture of said drag reducing polymeric material dispersed throughout said insoluble polymeric material, said insoluble polymeric material reinforcing said outer layer to provide a pliable integral outer layer, affixing said outer layer to said solid surface by use of a liquid insoluble pressure sensitive adhesive.

7. A method of claim 6, wherein said composite is affixed to solid surfaces which travel through saline solutions, fresh water or sea water.

8. A drag reduction composite substantially as hereinbefore described

9. A method for reducing drag substantially as described in accordance with Example 1 herein.