A protective coating

Abstract: The present invention provides an article having a coating comprising a first layer (2) of a fusion bond epoxy resin bonded to the article (1), a layer (4) of polyolefin such as polyethylene bonded to the first fusion bond epoxy resin (2) by a first adhesive (3), and a second layer (6) of a fusion bond epoxy resin bonded to the polyolefin (4) by a second adhesive (5). Since the second epoxy resin (6) is applied over the polyethylene layer (4) an increased thickness of the polyethylene is achievable thereby increasing the shock resistance of the housing on impact with an external body.

Description: Field of the Invention This invention relates to an article having a coating for protecting the article from damage (eg. by corrosion and/or by abrasion) and for providing visual evidence of impact, and to a method for applying the coating. The invention has particular application in the protection of housings to be used in an underwater (submarine) environment. Background to the Invention Submarine housings are exposed to extreme environmental stresses during use. Such stresses reduce the service life of submarine housings. Steel is regarded as the industry standard for submarine housings. However, particularly in a marine environment, steel is sensitive to chemical attack (eg. by electrolytic corrosion which creates hydrogen) and to physical damage (eg. by abrasion). One attempt to minimise the above problems associated with steel housings has been to employ a beryllium/copper housing. However, this material is very expensive. It also has serious disadvantages in terms of mechanical performance and it is environmentally unfriendly. These factors make beryllium/copper an unsuitable substitute for steel. Conventional protective coatings employ a sandwich structure of an epoxy resin layer, this being applied to the steel housing, followed by application of an adhesive and then a polypropylene layer. An alternative system employs the use of polyethylene instead of polypropylene. The abrasion resistance of these systems is inadequate. In the above-described conventional protective coatings, a polyolefin forms the outer layer of the coating. However, polyolefins have elastic properties. As a result, exposure of a coated housing to a physical shock (eg. an impact) is rarely detectable since the polyolefin layer returns to its original topography. With such coated housings, it is not possible to detect a potentially critical crack in the underlying epoxy layer. The inability to detect a damaged epoxy layer poses a significant problem. Submarine housings often contain very expensive and delicate electronic systems such as optical repeaters (electronic components which amplify an electronic signal) or branching units (components permitting a cable to branch into two or more cables). Exposure of such apparatuses to a physical shock, for example above a specified gravitational force, may be critical. Installation of a damaged housing and/or a damaged apparatus within a submarine housing is clearly undesirable. This is of great significance in a submarine environment where, following deployment, subsequent access may be severely restricted. Another problem associated with conventional protective coating systems is that the outer polyolefin layer is soft and therefore susceptible to abrasion damage (eg. ripping and/or tearing). This could occur, for example, during transport of the coated article. In the case of a submarine housing, this may occur in particular during deployment of the housing from a ship, during which linear engines and mechanical guides are employed. A yet further problem associated with conventional protective coating systems is that, to date, it has not proved possible to effectively adhere a paint to the outer polyolefin layer. It is desirable to firmly adhere a paint to an article for a number of reasons. A particular colour may be required for technical reasons, for example to help reflect light and/or heat, or conversely to absorb light and/or heat. In the technical field of submarine housings, international/national regulations may require a specific colour. In addition, the outer coating is held between a wheel pair and its frictional characteristics are important for grip. Damage to the soft outer polyolefin layer may therefore cause subsequent slippage during deployment. Alternatively, the end user may request a specific colour. Polyolefins are notoriously difficult to adhere a paint thereto. Conventional attempts have employed a primer, which is applied to the outer surface of the polyolefin, followed by application of a paint. Whilst such attempts have resulted in a painted article, the paint is not firmly adhered to the polyolefin with the result that the paint together with the primer are readily chipped off following an impact. There is therefore a need for an epoxy/polyolefin coating system which may be readily painted and where the paint is firmly adhered to the polyolefin layer. A further problem is that an outer polyolefin layer is not suitable for engraving (eg. for inscription of the manufacturer's or buyer's name). This same problem also exists with an outer coating comprising the above-described primer/paint combination. There is therefore a need for an epoxy/polyolefin coating wherein the outer layer is suitable for engraving. Summary of the Invention According to a first aspect of the present invention, there is provided an article having a coating, said coating comprising a first layer of a fusion bond epoxy resin bonded to the article, a layer of polyolefin bonded to the first fusion bond epoxy resin by a first adhesive, and a second layer of a fusion bond epoxy resin bonded to the polyolefin by a second adhesive. The term "bonded" includes any chemical and/or physical interaction between two or more components which resists separation of said components. The term "fusion bond epoxy resin" is well-known in the art. A fusion bond epoxy resin is characterised by a high temperature cure and a high T g which in turn provide a high degree of hardness, a high degree of toughness, and a reduced production time (ie. a reduced cure time). In the coating of the present invention, the first epoxy layer protects the article from corrosion; the polyolefin layer provides a water-proofing and cushioning function, and protects the first epoxy layer from chemical attack and physical damage; and, the second epoxy layer primarily provides an abrasion resistance function and, in contrast to a polyolefin surface, it provides an excellent, clean surface for applying paint thereto and/or for engraving purposes. Compared with the conventional coatings, the coating of the present invention has unexpected and improved impact and corrosion resistances. These unexpected advantages are due to the second fusion bond epoxy resin. The outer epoxy layer, being non-elastic, provides a means for detecting an impact to the coated article as it prevents the polyolefin from springing back. By assessing the extent of a series of impacts a calibration curve may be prepared to provide a quick test for determining whether or not a casing and/or the components located within said casing would be acceptable for subsequent use. It should be noted that even an extreme impact is absorbed by the multi-layer coating, the integrity of the first epoxy layer and the protective function of the multi-layer coating thus being maintained. The coating is environmentally safe, meaning that, during use, environmental pollution (eg. by dissolution of part of the coating, resulting in exposure of the article) does not occur, or does not occur to any appreciable extent. The article is preferably a housing, for example, a pipe. More preferably, the article is a submarine housing, for example, a housing which has been adapted to encase a cable, an optical repeater, or a branching unit used in submarine communications systems. The housing may be made from any material which has the requisite strength property for the technical purpose in mind. It is preferably made from steel, more preferably from mild steel. The first and second layers of fusion bond epoxy resins may be the same or different. Each epoxy resin may be a one- or two-part system. Two-part systems are preferred because of their improved shelf-life. Preferred epoxy resins are those having a cure time (time to quench) at 235 °C of less than 2 min, preferably less than 90 sec, more preferably less than 70 sec, and most preferably less than or equal to 60 seconds. The preferred epoxy resins are those of the VALSPAR range (supplied by Jotun, UK). The most preferred epoxy resin is VALSPAR D2003DV. Each epoxy resin layer is preferably applied at a thickness of up to 1 mm, more preferably at a thickness of between 50 µm and 500 µm, most preferably at a thickness of between 350 µm to 450 µm. The thickness of the epoxy layer is important to ensure that acceptable heat dissipation from within the article is achievable. The thickness of the second epoxy layer is particularly important to ensure a smooth outer surface for painting and/or engraving. The thicker the epoxy layer is, the more prone it would be to cracking. In a particularly preferred example, the second epoxy layer is thinner than the first epoxy layer. Most preferably, the first epoxy layer is 340-350 µm thick and the second epoxy layer is 100-200 µm thick. The first and second adhesive may be the same or different. The adhesives of the present invention include any adhesives which are capable of bonding together a fusion bond epoxy resin and a polyolefin. The adhesive does not form a distinct layer between the fusion bond epoxy resin and the polyolefin layers, but instead diffuses into each layer (in particular, into the epoxy layer) during application of the multi-layer coat. This is known as formulation of the adhesive, which takes account of the properties of both the epoxy and polyolefin, and provides a strong bonding between the two layers. Preferred adhesives in the present invention are those known in the art as "hot melts". These adhesives are characterised by their ability to be applied to a surface at a high temperature, preferably at a temperature in excess of 150 °C, more preferably in excess of 180 °C, particularly preferably in excess of 200 °C, and most preferably in excess of 250 °C. A high application temperature tolerance improves diffusion and formulation of the adhesive into the epoxy and polyolefin layers. The preferred adhesives preferably have a DSC melting point of greater than 120 °C, more preferably greater than 125 °C, and most preferably greater than 135 °C. The preferred adhesives are those of the FUSABOND range supplied by Jotun, UK. The most preferred adhesive is FUSABOND E MB 328DG. The first and second adhesives are preferably applied at a thickness of between 10 and 400 µm, more preferably between 30 and 200 µm, and most preferably between 50 and 100 µm. Preferred polyolefins are polypropylene and polyethylene. Polyethylene is most preferred since it is less susceptible to cracking than is, for example, polypropylene. Polyethylene embrittles at approximately -50 °C, whereas polypropylene embrittles at approximately -5 °C. A further advantage of polyethylene is that, following casting, a relaxing period for allowing expansion and contraction is not required. The polyolefin layer is preferably 1 to 6 mm thick, more preferably 2 to 4 mm thick, and most preferably 2.5 to 3.3 mm thick. The preferred polyolefins are those having a density (ASTM method D792) of between 0.7 and 1.2 g/cm 3 , more preferably between 0.8 and 1.1 g/cm 3 , and most preferably between 0.9 and 1.0 g/cm 3 . A shore "D" hardness (ASTM method D2240) of at least 50, more preferably at least 60, is preferred. The preferred polyolefins are those of the NOVAPOL range supplied by Jotun, UK. The most preferred polyethylene is NOVAPOL TD-2106 BKG. A paint layer may be applied to the second epoxy resin layer. The paint is preferably a polyurethane- or an epoxy-based paint. Such paints provide an additional water protection function. The paint may be a one- or two-part paint system. In the latter case, a hardener would be required. Where appropriate, a thinner may also be employed. Suitable paints for the purpose of the present invention are well-known to the skilled person and have been widely used, for example, in aerospace technology. Two-part systems typically contain synthetic resins and organic/inorganic pigments in organic solvents. Certain colours may contain lead based pigments. When mixed with a hardener/curing agent, the blend typically contains isocyanates. The above paints are readily available from a number of commercial sources, eg. Trimite Limited, Uxbridge, Middlesex, UK. A particularly preferred paint is the two-part system supplied by Trimite Limited which comprises the polyurethane base component B93 (for finishes to specification DTD5580A), the hardener J9301 (for finishes to specification DTD5580A and DTD5618), and the thinner BT93. According to a second aspect of the present invention, there is provided a method for applying a coating to an article, comprising the steps: applying a layer of a first fusion bond epoxy resin to an article; applying a first adhesive to the first fusion bond epoxy resin layer; applying a layer of a polyolefin to the first adhesive; applying a second adhesive to the polyolefin layer; and, applying a second fusion bond epoxy resin to the second adhesive. Preferably, the method steps are performed sequentially and in the order as defined above. Each epoxy resin layer, and/or each adhesive, and/or the polyolefin layer are preferably applied by spraying. Spraying provides a simplified method for applying the coating. The article is preferably pre-heated prior to application of the coating. This is to ensure that a minimum temperature is maintained during the subsequent application of the first and/or second epoxy resin, the first and/or second adhesive, and/or the polyolefin. The preferred minimum temperature of pre-heating is 180 °C, more preferably 200 °C, particularly preferably 220 °C, and most preferably 240 °C. The most preferred working range is 232 °C to 246 °C. The article is preferably re-heated prior to the application of each component of the multi-layer coat. Again, this is to ensure that a minimum temperature is maintained during application of the component in question. Each epoxy resin layer is preferably applied at a minimum temperature of 180 °C, more preferably 200 °C, particularly preferably 220 °C, and most preferably 240 °C. Each adhesive component is preferably applied at a minimum temperature of 180 °C, more preferably 200 °C, particularly preferably 220 °C, and most preferably 240 °C. The polyolefin layer is preferably applied at a minimum temperature of 180 °C, more preferably 200 °C, particularly preferably 220 °C, and most preferably 240 °C. In a preferred example, the article is pre-treated prior to application of the coating. Where the article is made, for example, from steel, the article may be grit-blasted and/or treated with chromate. When inspected after grit blasting (blast cleaning to SA3 standard), the housing surface preferably has a mean peak-to-valley profile range of 25-200 µm, most preferably of 50-100 µm (surface standard BS 7079 Part A1, supplement 1, 1989). When inspected after blast cleaning, the estimated surface debris is preferably not greater than a quantity rating 1, class 2 (ISO 8602-3: 1992-10-01- draft BS 7079: part B3). In a preferred example, the second adhesive is applied over a substantial part (preferably the whole) of the polyolefin layer, preferably in a single application. Preferably, the second epoxy resin layer is applied over a substantial part (preferably the whole) of the second adhesive, and preferably in a single application. This helps to provide a smooth and clean surface for subsequent painting. Brief Description of the Drawings The present invention will now described by reference to the following example and accompanying drawing, in which Figure 1 shows a coating applied to a portion of the casing for an article to be used in a submarine environment. Detailed Description Example A steel submarine housing 1 for an optical repeater was blast cleaned to SA3 standard (BS 7079, part A1, supplement 1, 1989), having a estimated surface debris quantity rating of less than 1, class 2 (draft BS 7079: part B3). The blast protection rings were removed, and a chromate solution (Comet PC solution) was applied by brush and allowed to dry. Care was taken to ensure that there were no drips or runs on the lower face. Immediately prior to application of the multi-layer coating, the appearance of the chromate film was visually assessed. A light to dark, golden brown colour with a tightly adherent film and no flaking or heavy "rings" evident constitutes an acceptable chromating treatment. a) Multi-layer coating composition: The Triple Polyethylene (PE) NOVAPOL TD2106BK system was employed. This conventional multi-layer system comprises the three following components: (i) as the first fusion bond epoxy resin 2 - FBE D2003DV; (ii) as the first adhesive 3 - FUSABOND E MB 328DG; and (iii) as the polyolefin 4 - NOVAPOL polyethylene TD2106BK. To this conventional multi-layer coating, the following two components were then applied (the above application temperatures were employed): (iv) as the second adhesive 5 - FUSABOND E MB 328DG; and (v) as the second fusion bond epoxy resin 6 - FBE D2003DV. b) Application of the multi-layer coating: The housing 1 was heated to 240-250 °C and the first epoxy resin layer 2 applied thereto at a thickness of between 350 and 400 µm. As soon as the thickness has been established to be correct, a further thin coat (approximately 50 µm) of the first epoxy resin 2 was then applied to produce an overall maximum thickness of approximately 500 µm. Immediately thereafter, a thin coat of first adhesive 3 (approximately 50-100 µm) was applied over all of the first epoxy resin, followed by application of the polyethylene layer 4 (approximately 2.5 to 3.3 mm), the polyethylene 4 being applied by multiple passes as evenly as possible. The partially coated housing was turned clockwise through 45° after each coat, except for the last coat. The housing was returned to an oven when the temperature dropped to 160 °C, or when the applied polyethylene ceased to flow. This ensured a minimum application temperature of 232-246 °C. The second adhesive 5 (approximately 50-100 µm thickness) and second epoxy resin 6 (approximately 100-200 µm) were applied under the same process conditions as were the first adhesive and first epoxy resin. , Since the second epoxy resin 6 is applied over the polyethylene layer 4, an increased thickness of the polyethylene is achievable. Without the second epoxy resin layer 6, the polyethylene layer sags creating an uneven, ridged coating. The use of the epoxy resin layer 6, enables an increase in the polyethylene layer without encountering these problems. This is clearly desirable since the greater the thickness of the polyethylene layer, the greater the shock resistance of the housing on impact with an external body. Presently, thicknesses of 4mm of a polyethylene have been achieved using this structure and manufacturing method although greater thicknesses are also possible. Finally, a paint coating 7 was applied. The chosen paint was a two-part Trimite paint system comprising a polyurethane base (B93) and a hardener (J9301). Testing of the coating 1 Impact testing The coated housing was subjected to various local impact loads. In particular, following testing in accordance with British Gas Standard CW6 (1 kg at 15 cm height), only an indent was observed in the surface of the second fusion bond epoxy resin layer 6. At a height of 100 cm and using a 1 kg weight (approximately 7 times the Gas Standard), the second fusion bond epoxy resin layer 6 cracked and the polyethylene layer 4 dented. However, the cracked epoxy resin 6 was still bonded to the polyethylene 4. On removal of the second epoxy layer, achieved by cutting through the polyethylene below the cracked second epoxy layer, the polyethylene showed no signs of cracking or debonding. At higher impact loads (namely, assaulting the coated housing with a hammer), a piece of the coating was debonded at the second epoxy resin/polyethylene interface leaving behind a first epoxy resin-coated steel housing. 2 Pressure test The coated housing was subjected to a pressure test of 83 MPa for a period of 1 week, with a depressurisation time of 2 hours ± 15 mins. The coated article passed the visual inspection and detection test, both before and after the above pressure test. The coating also resisted efforts to lift it using a screw driver. H.V. tests on the coated housing showed no signs of failure on the coating at 15 kV. 3 Temperature test The coated housing was subjected to an extended temperature cycle test of 10 cycles of -25±3 °C to +47±5 °C, with a dwell time of 1 hour at each temperature and a 30±5 minute change over period between extremes. A visual examination following the post-temperature cycle revealed that no degradation or debonding of the coating had taken place. 4 Compatibility test The coated housing was immersed in various solutions with which it may come into contact with during its service life. The coated housing was immersed separately at room temperature and at 50 °C for 1 week. The following solutions were employed: Markam Fluid (a white spirit substitute); Insogel; Energrease, sea water; and petroleum jelly. The second epoxy resin layer 6 softened following exposure with Markam Fluid (both at room temperature and at 50 °C), but it did not debond. No reaction occurred with any of the other solutions. 5 Engraving test Engraving was performed on the coating. The paint did not flake or peel and the lettering produced was sharp in appearance. 6 Electrical test Electrical spark testing was carried out at 15 kV DC. The coated housing passed the spark tests.

Claims: An article having a coating, the coating comprising a first layer (2) of a fusion bond epoxy resin bonded to the article (1), a layer (4) of polyolefin bonded to the first fusion bond epoxy resin (2) by a first adhesive (3), and a second layer (6) of a fusion bond epoxy resin bonded to the polyolefin (4) by a second adhesive (5). An article according to claim 1, in which the article (1) is a submarine housing. An article according to claim 1 or 2, in which the article (1) is an optical repeater, a branching unit, or a cable. A method for applying a coating to an article, comprising the steps: applying a layer of a first fusion bond epoxy resin to an article; applying a first adhesive to the first fusion bond epoxy resin layer; applying a layer of a polyolefin to the first adhesive; applying a second adhesive to the polyolefin layer; and. applying a second fusion bond epoxy resin to the second adhesive. A method according to claim 4, in which the first and/or second epoxy resin layer, and/or the first and second adhesive, and/or the polyolefin layer are applied by spraying. A method according to claim 4 or 5, in which the article is heated to a minimum temperature prior to application of the first and/or second epoxy resin layer, and/or the first and second adhesive, and/or the polyolefin layer. A method according to any of claims 4 to 6, in which the article is pre-treated prior to application of the multi-layer coating. A method according to any of claims 4 to 7, in which the second epoxy resin layer is applied, in a single application, over a substantial part of the second adhesive. A method according to claim 8, in which the second epoxy resin layer is applied in a single application. An article or method according to any preceding claim, in which the article (1) is made from steel. An article or method according to any preceding claim, in which the first (2) and second (6) fusion bond epoxy resins have a cure time (time to quench) at 235 °C of less than 2 minutes. An article or method according to any preceding claim, in which the first (2) and/or second (6) fusion bond epoxy resin layers have a thickness of up to 1 mm. An article or method according to any preceding claim, in which the first (3) and/or second (5) adhesive is a hot melt adhesive. An article or method according to any preceding claim, in which the first (3) and/or second (5) adhesive has a DSC melting point of greater than 120 °C. An article or method according to any preceding claim, in which the polyolefin (4) is polyethylene. An article or method according to any preceding claim, in which the polyolefin layer (4) has a thickness of 1 to 6 mm. An article or method according to any preceding claim, in which the polyolefin (4) has a density of between 0.7 and 1.2 g/cm 3 . An article or method according to any preceding claim, in which the polyolefin (4) has a shore D hardness of at least 50. An article or method according to any preceding claim, in which the coating has a further layer comprising a paint (7) which is bonded to the second fusion bond epoxy resin layer (6). An article or method according to claim 19, in which the paint (7) is a polyurethane- or an epoxy-based paint.

Inventors: GOULD SANDRA LOUISE, NORMAN PHILIP ANDREW, GOULD, SANDRA LOUISE, NORMAN, PHILIP ANDREW
Applicants: CIT ALCATEL, ALCATEL
Application year: 1999
Main classification: B05D7
IPC codes: B05D7, B05D1, B32B27