WO2004086147A2

WO2004086147A2 - Method of imparting patterns of surface relief to rigid substrates

Info

Publication number: WO2004086147A2
Application number: PCT/CA2004/000431
Authority: WO
Inventors: Winston G. Chand; C.N. Catherine Lam; Nicola A. Landreth; Cara L. Phillips; David K. Potter; Robert E. Steele
Original assignee: Shawcor Ltd.
Priority date: 2003-03-26
Filing date: 2004-03-23
Publication date: 2004-10-07
Also published as: CA2423531A1; WO2004086147A3

Abstract

A method of providing a pattern of surface relief on a rigid substrate. A layer of hardenable material is formed on the rigid substrate. The layer has the pattern formed in an exposed surface of the layer while it is non-hardened. The layer is hardened to form a hardened layer of the material having the pattern formed in its exposed surface, on the rigid substrate.

Description

METHOD OF IMPARTING PATTERNS OF SURFACE RELIEF TO RIGID SUBSTRATES

- 1 -

This invention relates to a method of imparting patterns of surface relief to rigid substrates. By the term "pattern" is meant an arrangement,, usually repeated, of similar elements, usually disposed at regular spacings. Examples of such patterns may include arrangements of ribs, grooves or embossments providing visually appealing or decorative textures, optical effects or friction increasing effects. Examples of substrates include articles to which it may be desired to impart such textures or effects, for example manufactured goods such as electrical appliances, ■tools, stands and step ladders.

Of especial interest are drag reduction patterns which may be applied to substrates such as airfoil, hydrofoil or inner pipe surfaces, which reduce the drag experienced by fluids flowing over these substrates. The use of a pattern of multiple substantially or approximately V-shaped grooves on hydrodynamic or aerodynamic surfaces to reduce skin friction has been known for some time. It is believed that such grooves, or "riblets", reduce the viscous drag by deliberately disturbing the laminar boundary layer in a way that the near wall turbulence structure is altered compared to the normal turbulent flow condition. The dimensions of the riblets required have been ascertained in various flow studies and are usually for example on the order of a few thousands of an inch (0.01 to 1 mm) in height and in peak to peak spacing. Drag reductions of 10% and more have been observed.

Methods previously described for imparting desired pattern to a substrate have included adhering to the surface flexible tapes or films to which the pattern has been imparted, as described in U.S. Patents Nos . 4,706,910 (Walsh et al) and 4,986,496 (Marentic et al) . Canadian Patent Application 2,151,170 (Weiss et al) describes cutting the desired pattern into the surface using a tool designed for the purpose.

However, the application of tapes and films presents practical problems of application of the tapes and films to the substrate, since they may not adhere well to the substrate under various installation conditions, and it may be difficult to properly align the tape or film, and there may be difficulties in accessing the substrate. The adhered tape or film may not withstand subsequent operations carried out on the substrate. For example, in the case of application to an inner pipe surface, the tape or film may not withstand the application of hot fusion bonded coatings such as epoxy coatings to the exterior surface. Further, the tape or film may not stand up well to service conditions such as weather conditions, temperature variation and fluid resistance. Further, there may be problems of fluid ingress at the seams between adjacent tape edges that may impair the bond between the tape and the substrate or may damage the substrate.

Further, the tapes and films impart extra weight ^• to the substrate. Further, the tapes, films and adhesives are relatively expensive, particularly if applied to large regular surfaces such as the inside of long, large-diameter gas transmission pipe.

Cutting a pattern into the surface of a substrate requires consideration of how the substrate will subsequently be protected without losing the pattern. Application of a coating over the patterned surface is likely to result in total or partial loss of the pattern definition due to pattern filing and flow out of the coating. In some applications the precut thickness of the substrate and the mechanical integrity during and after the cutting operation may be a concern. Alternatively the pattern could be cut into a protective coating on the substrate, however, this is likely to result in some loss of coating protection and the reproduction of a detailed pattern definition would again be difficult. In some cases, removal of the cuttings may be an issue. The economics and logistics of cutting the pattern on a large scale may also preclude the use of this technique.

In the present invention there is provided a method of providing a pattern of surface relief on a rigid substrate comprising forming on the rigid substrate a layer of hardenable material having said pattern formed in an exposed surface of the layer while it is non-hardened, and hardening the layer to provide on the rigid substrate a hardened layer of the material having the pattern formed in the exposed surface thereof.

Such method may be relatively easily and economically applied to the provision of a pattern on relatively large substrates. The method provides the further advantage that the hardened layer may provide a coating fulfilling the service requirements for a particular application. For example, the layer may be specifically designed to adhere to the substrate of interest and to have performance attributes consistent with the application such as chemical resistance and operating service temperature range. Further, the layer can be applied in such a way as to provide a seamless surface, while alignment of a pattern relative to a feature of the substrate, for example alignment of a drag reduction pattern, can be relatively easily accommodated by the manner in which the layer is formed, for example by alignment by a pattern application • tool .

The method of the invention provides the advantage that the hardened layer may provide a protective coating, thereby obviating the need to apply another coating over the pattern providing layer.

Various procedures are contemplated for providing the layer of hardenable material having the formed in an exposed surface of the layer on the rigid substrate.

In carrying out the present method, in some instances, the layer of hardenable material having the pattern formed in its exposed surface may be formed slightly before or simultaneously with the step of applying the hardenable material to the rigid substrate. For example, the layer having the pattern formed in it may be provided by extruding the hardenable material through a correspondingly profiled extrusion die that moves relative to the substrate, so that the extruded patterned layer is deposited on the substrate- at or shortly after its extrusion. In other procedures, a coating applicator, for example a squeegee having for example a comb-like profile, may be used to apply the hardenable material on the rigid substrate.

In one preferred embodiment of the present method, a two stage procedure may be followed for forming the layer of hardenable material having the pattern in it on the substrate. In such case, firstly a layer of the ^• hardenable material may be applied to the rigid substrate and, secondly, the layer of hardenable material formed on the substrate is provided with the riblets in its exposed surface. For example, a coating of substantially uniform thickness of the hardenable material may be formed on the rigid substrate using, for example, a draw down instrument such as a doctor blade device or the like, that provides a coating of a desired thickness and, subsequently, a pattern may be formed in the surface of the coating by engaging the surface with a profiling tool that is moved laterally along the surface of the hardenable material layer. Various forms of tool are contemplated. For example, the tool may comprise a comb with a row of teeth that may be dragged through the hardenable coating layer to form grooves in its surface. Alternatively, the tool may be a brush ■ having protruding bristle formations that may be disposed in more than one row and may be staggered to achieve a desired spacing and depth and height dimensions to the pattern. In other examples, the tool may be a grooved or threaded roller that may be dragged or rolled over the surface of the hardenable material layer to form the pattern.

While various forms of hardenable material composition are contemplated, such as plastisols and organosols that may be hardened by, for example, application of heat, in the preferred form the hardenable material composition comprises a curable resin.

As applied to the substrate, the curable resin composition may contain a curing agent that is mixed into the resin shortly before application to the substrate. In such case, the resin and curing agent may be selected so that the curing time is sufficiently short that, once the pattern is formed, the resin hardens before there is appreciable loss of definition of the pattern. Alternatively, the curable resin composition may be exposed to a curing agent after the pattern is formed in its surface. While application of liquid or, more preferably, gaseous curing agents is contemplated, in the most preferred form the curable resin is a radiation curable resin and the step of hardening the layer comprises exposing the layer to curing inducing radiation. Ultraviolet light (UV) curable coatings are particularly preferred because of the rapid polymerization of the UV curable compositions.

In one preferred procedure, the profiling tool comprises a grooved or profiled UV transparent roller in which is contained a UV light source. The roller is moved over a layer of the curable resin composition applied on the substrate. As the roller moves over the coating surface, the pattern is imparted and cure is initiated at the same time.

In particularly preferred embodiments of the invention, a drag reduction pattern is formed on an inner surface of pipes and like conduits by applying a UV curable resin composition coating on the inner surface, forming a drag reduction pattern in the surface of the coating, for example riblets aligned substantially parallel to the direction of flow of fluid through the pipe ^■ or like conduit, and curing the composition substantially simultaneously with or substantially immediately after forming the pattern.

In further preferred embodiments, drag reduction patterns are formed as described above on airfoils, boat hulls, race car bodies, sports helmets and like articles that in use react with a fluid flowing relative to them to exert a drag force.

In some cases, in order to ensure that, as is desirable, initiation of cure of a curable resin occurs within a time frame that is short enough to limit pattern loss by flow out of the uncured patterned coating, the rheological properties of the coating may be adjusted by addition of viscosity enhancing agents or thixotroping agents that are compatible with the resin composition and do not interfere with the curing mechanism or impair the desired chemical and physical properties for • the cured product. Further, since the viscosity and thixotropy of the composition may vary according to the application temperature, the rheology of the coating may need to be adjusted dependent on the application temperature by altering the coating viscosity and thixotropy in order to allow an adequate period for cure of the composition before undesirable loss of the pattern occurs to an undesirable extent.

Although the above description provides ample information for one of ordinary skill in the art, to carry out the present method, for the avoidance of doubt, a detailed example will be provided.

EXAMPLE

A riblet pattern was formed on a metal plate.

Liquid UV curable coating compositions numbers 1 and 2 were prepared by mixing together the ingredients identified in Table 1 below in the parts by weight indicated. TABLE 1

Ingredient Composition

Notes

1. ex. Sartomer, Exton, Pennsylvania, USA

2. ex. Ciba Specialty Chemicals, Mississauga, Ontario, Canada

3. ex. Nyco Minerals, Inc., Willsboro, New York, USA

4. ex. Degussa, Theodore, Alabama, USA

5. ex Byk Chemie USA, Wallingford, Connecticut, U.S.A.

6. ex. Suncolor Corporation, North Canton, Ohio, USA

For each composition, metal plates were coated with the liquid composition using a draw down instrument to provide a uniform thickness layer of 8 to 10 mils (0.2 to 0.25 mm) .

Riblets were formed in the coating by dragging a metallic comb (a metal comb used for treatment of head lice) manually through the liquid coating in order to form grooves in the coating with the teeth of the comb.

The substrate with the ribleted coating was passed under a source of UV radiation. A Fusion UV LC-6 conveyor equipped with an F300S lamp system (available from Fusion UV Systems Inc., Gathersburg, Maryland, U.S.A.) was used with a "D" bulb focused to the top of the substrate. The substrate was placed on the conveyor and passed under the UV lamp at 5 ft./min. (1.52 m./min.). The riblet dimensions of the cured and hardened resin patterned coating were examined and were found to be as follows:

Peak to peak distance 30 to 37 mils (0.8 to 0.9 mm.)

Peak height (measured from interface between the plate and the cured resin) 10 to 16 mils (0.25 to 0.4 mm.)

Groove depth 2 mils (0.05 mm.)

It was found that, after forming the riblets in the uncured resin composition coating, the uncured coating could be left uncured for at least 3 minutes while retaining riblet definition.

While the above detailed Example has described formation of a riblet pattern the method of- the present invention is by no means limited to riblet .formation and can be used in formations of patterns of surface relief that comprise, for example, raised elements such as ribs or embossments on a surface. Such surface may be, for example, a generally planar surface, or a simply curved for example a plane concave or convex surface, of a hardened layer. Further, the relief may comprise depressions in such generally planar or simply curved surface. Such depressions may comprise regularly shaped depressions disposed at regular intervals, for example regularly spaced depressions that are circular or of some other geometric form.

Claims

CLAIMS :

1. A method of providing a pattern of surface relief on a rigid substrate comprising forming on the rigid substrate a layer of hardenable material having said pattern formed in an exposed surface of the layer while it is non-hardened, and hardening the layer to provide on the rigid substrate a hardened layer of the material having the pattern formed in the exposed surface thereof.

2. A method of providing a pattern of surface relief on a rigid substrate according to claim 1 including a step of providing said layer of hardenable material having said pattern formed in an exposed surface of the layer before or simultaneously with applying said hardenable material to said rigid substrate.

3. A method of providing a pattern of surface relief on a rigid substrate according to claim 2 wherein said step of providing said layer of hardenable material having said pattern formed in an exposed surface of the layer comprises extruding said hardenable material through an extrusion die.

. A method a providing a pattern of surface relief on a rigid substrate according to claim 2 wherein said step of

■ providing said layer of hardenable material having said pattern formed in an exposed surface of the layer comprises applying said hardenable material to said rigid substrate by coating applicator having a profiled edge corresponding to a desired profile of said pattern.

5. A method of providing a pattern of surface relief on a rigid substrate according to claim 1 including a step of providing said layer of hardenable material having said pattern formed in an exposed surface of the layer after applying a layer of said hardenable material to the rigid substrate .

6. A method of providing a pattern of surface relief on a rigid substrate according to claim 5 wherein said step of providing said layer of hardenable material having said pattern formed in an exposed surface of the layer comprises forming a coating of substantially uniform thickness of said hardenabie material on said rigid substrate and forming said pattern in a surface of said coating by engaging said surface' with a profiled tool and moving said tool laterally along said surface.

7. A method of providing a pattern of surface relief on a rigid substrate according to claim 1 wherein said hardenable material comprises a curable resin and said step of hardening the layer comprises exposing the layer to a curing agent.

8. A method of providing a pattern of surface relief on a rigid substrate according to claim 7 wherein said curable resin comprises a radiation curable resin and said step of hardening the layer comprises exposing the layer to cure inducing radiation.

9. A method of providing a pattern of surface relief on a rigid substrate according to claim 8 wherein said cure inducing radiation is UV radiation.

10. A method of providing a pattern of surface relief on a rigid substrate according to claim 1 wherein said rigid substrate comprises a tubular pipe having an inner surface and said pattern is provided on said inner surface.

11. A method of providing a pattern of surface relief according to claim 1 wherein the hardenable material comprises a viscosity enhancing agent and/or a thixotroping agent compatible therewith.

12. A method according to claim 1 wherein said surface relief comprises raised elements on a surface.

13. A method according to claim 1 wherein said surface relief comprises depressions in a generally planar surface.

14. A method according to claim 1 where.in said, surface relief comprises grooves.

15. A method according to claim 1 wherein said surface relief .comprises V-shaped grooves.

16. A method according to claim 1 wherein said surface relief comprises riblets .