This invention relates to a method of protectively enclosing elongate substrates, especially, for example, pipelines, and to a protective sheet for use in such a method, the term "sheet" including elongate sheets of generally "tape-like" form.

Metal pipelines can be protected against corrosion by means of coatings applied to sections of the pipe before delivery to the construction site, but such coatings are often imperfect or may be damaged during transport and installation of the pipe sections. Protective wrappings of polymeric sheeting can be adhered to the pipeline in situ, using mastics, hot melt adhesives, or heat curable epoxies, after assembly of the pipeline sections, thus avoiding some of the problems of the factory-applied coatings. However, these methods -often fail to provide a bond between the sheeting and the pipeline which will withstand conditions encountered in service on the pipeline, such as temperature cycling, elevated tem- peratures, and especially soil stress caused by expan¬ sion and contraction of buried pipelines. The present invention introduces an improved method and sheet which have significant advantages for protectively enclosing elongate substrates such as pipelines.

The invention provides a method of protectively enclosing an elongate substrate wherein a sheet of polymeric material is adhered around the substrate by arranging between the sheet material and t'he substrate a multi-component adhesive comprising a first component (a) comprising a substantially solid curable mixture of a free-radical-curable monomer, a polymer and a free-

radical generator, and a second component (b) compris¬ ing a catalyst for the free-radical curing of at least the monomer in mixture (a), and bringing the mixture (a) and the catalyst (b) into contact so as to bring about curing of the mixture (a).

In another aspect, the invention provides a dimensionally recoverable sheet of polymeric material carrying on one of its main surfaces (a) a substan¬ tially solid curable mixture of a free-radical curable monomer, a polymer and a free-radical generator, for use with (b) a catalyst for the free-radical curing of at least the monomer in mixture (a) in a method of enclosing an elongate substrate in the sheet; or carrying the catalyst (b) for use with mixture (a) in such a method.

Preferably, the method comprises (1) applying either the component (a) or the component (b) to the substrate, (2) enclosing the substrate in the polymeric sheet material carrying the other of the components(s) (a) and (b) such that the other of the components (a) and (b) is between the sheet material and the substrate, and (3) bringing the components (a) and (b) into contact with each other between the sheet material and the substrate so as to bring about curing of the mixture (a), thereby adhering the sheet material to the substrate.

It will be understood that "substantially solid" as used herein means that the mixture has sufficiently high viscosity to permit it to be pre-coated on the sheet or substrate without running off to any signifi¬ cant extent in contrast to the behaviour of a liquid

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(at ambient temperatures) monomer alone. Pre-coating on the sheet is preferred, in which case the mixture will preferably be "storage-stable" in the sense that it does not undergo any significant degree of curing at ambient or normal storage temperatures, thus enabling a precoated sheet product to be provided for convenient use in remote locations such as construction sites.

The term "curable" is understood to have its usual significance in that a "curable" mixture is capable of undergoing a chemical transformation (such as cross- linking or polymerising) resulting in a product which no longer flows appreciably under the combined effects of pressure and temperature greater than ambient.

The method and sheet according to this invention can be used for protecting any elongate substrate, for example splices in power or telecommunication cables, and is advantageously used on pipelines. The method is especially useful on metal pipelines, e.g. oil pipe¬ lines, since a cured adhesive bond can be achieved without deliberate mixing of the curing components between the substrate and the polymer sheet. Such a bond tends to have superior resistance to soil stress and other environmental effects and can be achieved despite the considerable heat sink effect of the pipeline, which inhibits curing of other curable adhesive formulations, especially when the pipeline is in operation conveying fluids, often at temperatures of not more than 80 ^β C, for example 40 ^β C. It has surpris¬ ingly been found that acceptable bonding can be achiev- ed by suitable embodiments of the present invention to polymer sheets based on polyolefins and between such sheets and metal substrates such as pipelines, despite the great dissimilarity of their respective surface properties and the known difficulty of achieving good adhesion to polyolefins.

The method may include the further step of burying the enclosed pipeline in the ground.

The curable monomer/polymer mixture preferably comprises a flowable liquid or semi-l iquid acryl ic monomer, which may mean so-called oligomers for example polyethylene glycol dimethacrylate or diacrylate, tripropylene glycol diacrylate, 1 ,6-hexane diol dimeth¬ acrylate or diacrylate, polytetramethylene ether glycol diacrylate, polybutadiene diacrylate, polyester-meth- acryl ate d imethacrylate, vinyl terminated acryl- onitrile-butadiene, acrylated epoxidised soyabean oil, trimethylol propane trimethacrylate; and a non-reactive polymer, preferably an acrylic polymer, in sufficient amount and having sufficient viscosity to render the mixture with the flowable monomer substant ially solid under ambient storage conditions . The free- radical generator is preferably one whose rate of free-radical generation in the mixture is catalysed by an amine preferably an aryl alkyl alkyl amine, for example an organic peroxide such as benzoyl peroxide, and the catalyst is preferably an amine . However, other catalysts could be used, for example transition metal salts such as cobalt naphthenate.

The acrylic polymer referred to above is thermo- plastic and may also be elastomeric. For example, polybutyl methacrylate (e.g. Elvacite 2044 , Du Pont , or

Plexigum P24 , Rohm) . (These are thermoplastics with a

Tg around or above room temperature) .

Non-acrylic polymers may be used , e. g. a vinyl acetate - ethylene copolymer, (e.g. VAE711 , Wacker, 70% vinyl acetate) . This is an elastomer.

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Other thermoplastic materials may be useful, e.g. ethylene-vinyl acetate copolymers (EVA) including less than 50% VA, ethylene-ethylacrylate copolymers (EEA), polyethylene-butyl acrylate.

A plasticiser may also be used if required, e.g. N-ethyl-o-,p-toluenesulphonamide (Santicizer 8, Mon¬ santo) or 2-ethyl hexyl diphenyl phosphate (Santicizer 141 , Monsanto) .

The polymer/monomer mixture may be coated on the substrate at the time when the sheeting is to be applied thereto, in which case the catalyst could be applied to the mixture on the substrate immediately before application of the polymer sheet thereto, but it may be preferable for the sheet to be pre-coated with the catalyst to promote adhesion to the sheet. When the sheet is pre-coated with the mixture, the catalyst could be applied to the sheet before application to the substrate, but the catalyst is preferably applied to the substrate, in which case it may be applied in a composition incorporating corrosion inhibitors or other agents to enhance the life of the substrate. The monomer/polymer mixture may be applied to the catalyst- coated substrate immediately before the sheet is applied but is preferably pre-coated on the sheet, in which case suitable surface treatments or primers may be applied to the sheet to promote adhesion thereto. The polymer/monomer mixture itself may incorporate materials for promoting adhesion, especially when polyolefin-based sheet is used.

The curing of the free-radical curable monomer proceeds upon surface-to-surface contact of the mon¬ omer/polymer mixture with the catalyst but may if desired be enhanced by application of heat, preferably sufficient to cause the curable mixture to soften or melt and enhance its surface wetting ability. It is an advantage of the present invention that curing on heat-sink substrates can be surprisingly complete, apparently due to catalytic curing proceeding from the inside together with heat curing proceeding from the outside of the adhesive layer, neither one of these mechanisms alone being sufficient to produce reliable curing throughout relatively thick adhesive layers.

The polymeric sheet is preferably dimensionally recov- erable, especially heat-recoverable, about the sub¬ strate to compress the curing components.

A heat recoverable article is an article the dimensional configuration of which may be made substan¬ tially to change when subjected to heat treatment.

Usually these articles recover, on heating, towards an original shape from which they have pre¬ viously been deformed but the term "heat-recoverable", as used herein, also includes an article which, on heating, adopts a new configuration, even if it has not been previously deformed.

In their most common form such articles comprise a heat-shrinkable sleeve made from a polymeric material exhibiting the property of elastic or plastic memory as described, for example, in U.S. Patents 2,027,962; 3,086,242 and 3,597,372. As is made clear in, for

example, U.S. Patent 2,027,962, the original dimen- sionally heat-stable form may be a transient form in a continuous process in which, for example, an extruded tube is expanded, whilst hot, to a dimensionally heat-unstable form but, in other applications, a preformed dimensionally heat stable article is deformed to a dimensionally heat unstable form in a separate stage.

In the production of heat recoverable articles, the polymeric material may be cross-linked at any stage in the production of the article that will enhance the desired dimensional recoverability. One manner of producing a heat-recoverable article comprises moulding the polymeric material into the desired heat-stable form, subsequently cross-linking the polymeric mat¬ erial, heating the article to a temperature above the crystalline melting point or, for amorphous materials, the softening point, as the case may be, of the poly¬ mer, deforming the article and cooling the article whilst in the deformed state so that the deformed state of the article is retained. In use, since the deformed state of the article is heat-unstable, application of heat will cause the article to assume its original heat-stable shape.

In other articles, as described, for example, in British Patent 1,440,524, an elastomeric member such as an outer tubular member is held in a stretched state by a second member, such as an inner tubular member, which, upon heating, weakens and thus allows the elastomeric member to recover.

Th e expre ss ion " d imens ional ly recoverabl e" , however , includes articles which are recoverable upo n sub j ect ion to st imul i other than heat , for example solvents , or impact weakening of a second "hold out" member, which may be useful as alternatives to heat-recoverable articles, for example in circum¬ stances where heating, especially flame heating, may be undesirable.

A polymeric composition may be cross-linked either by the incorporation of a chemical crosslinking agent or by exposure to high energy radiation. Examples of suitable cross-linking procedures are well known nowadays from numerous patents in the name of Raychem Corporation, and need not be further described here.

The degree of cross-linking of the compositions may be expressed in terms of the gel content (ANSI/ASTM D2765-68 ) of the cross-linked polymeric composition, i. e . exclud ing non-polymeri c additives that may be present. Preferably the gel content of the cross- linked composition is at least 10% , more preferably at least 20% , e.g. at least 30% , more preferably at least 40% .

While the sheet may be in the form of a tube, it is preferably in the form of a "wrap-around" sheet to be wrapped around the substrate and suitably fastened.

For pipel ines and other rel atively straight substrates, the sheet material preferably is recover¬ able to an extent of at least 10% , more preferably 20 - 40% , and preferably not more than 50% of its unre- covered dimensions. A recovery of about 30% provides adequate compression to promote good sealing contact

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and cure when the sheet is initially wrapped closely around the substrate, preferably in a "cigarette wrap" configuration to minimise the length of overlapping sheet edge and thus minimise potential leakage paths, and will nevertheless allow an adhesive patch closure of the wrapped around ends of the sheet to remain in position after recovery of the sheet. Recovery of more than 50% tends to overcome the adhesive bond of such a closure, although higher recovery may be used if the sheet is initially wrapped more loosely around the substrate, of if necessary to accommodate irregular substrates.

Preferably enough heat is applied to cause the polymer/monomer mixture to melt sufficiently to wet the substrate, and it is precisely at that stage that the heat-sink effect of a metal substrate such as a pipe¬ line becomes greatest, thus inhibiting the curing of known epoxy systems which require elevated curing temperatures.

A preferred form of sheet carries the monomer/ polymer mixture on one of its main surfaces and the catalyst on its other main surface, at least in those areas which are to be overlapped by the mixture-coated surface in use, so promoting curing of the mixture in the overlap areas to minimise leakage and promote sealing and bonding. The catalyst can be applied to the required areas either during manufacture of the sheet or subsequently, for example, immediately before or during installation on the substrate.

Methods of producing the polymeric sheet material carrying the appropriate curing components) can

readily be devised by persons familiar with polymer extrusion and coating technology. It has, however, been found advantageous that the preferred acrylic polymer/monomer mixture can be prepared by standard mixing techniques using liquid monomer without added solvent to give an appropriate viscosity for coating and the mixture, without heating or other treatment, then gels and becomes substantially solid after coating onto a suitable polymer sheet. These gelled polymer- in-monomer curable mixtures produce unexpectedly superior curing performance when used to adhere dimensionally recoverable article to a substrate about which the article is recovered in use.

Known coating methods, doctor blade for example, may be used, preferably to give a finished solid coating of thickness within the range from 0.5 mm to 1.5mm. The catalyst can also be applied to the other surface of the sheet by standard techniques, for example spray or roller coating.

Modified mixing and coating methods may be re¬ quired for polymers such as the aforementioned VAE711, which require relatively high shear forces to dissolve them directly in the monomer, since such high shear rates may induce premature curing. For example, mixing with the aid of auxiliary solvents, and coating by applying pre-cast films of the mixture to the sheet, may be adapted to suit the requirements of the mat¬ erials in question according to criteria which will be readily determinable by persons skilled in the relevant technology.

Suitable polymer sheet materials are known, for example polyolefins such as polyethylene, ethylene- vinyl acetate copolymers (EVA), polyvinyl chloride, polyethylene blended with EVA, or with ethylene-acrylic acid copolymers (EAA), ethylene-ethyl acrylate copoly¬ mers (EEA), or ethylene-propylene copolymers.

The curing components can be selected from known monomers, polymers, free-radical generators and cata¬ lysts and formulated to give a curing system having adequate shelf life, curing speed, and cured adhesion characteristics. Any polymer may be used which is compatible with the curable monomer in the sense that a pre-coatable (that is, substantially non-flowable at ambient or storage temperatures) mixture can be pre- pared in which the polymer does not unacceptably affect the curing. Any unsaturated monomer, free-radical generator and catalyst may be used which produce the desired curing reaction on contact between the solid coating containing the monomer (and free-radical generator) and the catalyst. Acrylic monomers are preferred, by which is meant acrylic and methacrylic acids and mono-, di-, and multi- (meth) acrylates, e.g. 2-ethylhexylacrylate, tetraethylene glycol dimeth¬ acrylate, trimethylol propane trimethacrylate.

In this connection, it is important that reactive polymers which normally incorporate a certain amount of stabiliser to promote storage stability should not contain such a level of stabiliser as would unaccept¬ ably inhibit or stop the curing reaction. For example, commercially available polyisobutylene and polypropy¬ lene oxide - alkyl glycidyl ether have been found to inhibit curing of acrylic monomers, whereas polybutyl ζffcEAC O K

methacrylate does not , possibly due to its having a lower level of stabiliser or possibly due to incompat¬ ibility or too high viscosity of the other polymers. Preferred polymers for use with the preferred acrylic monomers include poly (meth) acryl ates , especially polybutylmethacrylate, and vinyl acetate-ethylene copolymers such as the aforementioned VAE711.

The free-radical generator/catalyst system may be selected according to known criteria to suit the other materials and conditions of use. Peroxide free-radical generators, e.g. benzoyl peroxide, are suitable, and amine catalysts such as amines, for example N,N-di- methyl-p-toluidine or its saccharin salt, N-phenyldi- ethanolamine , may be used and/or catalysts such as organic compounds of metals of vari able oxidation state, e.g. iron, cobalt , or manganese salts (such as cobalt naphthenate ) or copper acetylacetonate or vanadium salts may be used.

The catalyst may be mixed with suitable carriers, binders, corrosion inhibitors, fillers , etc. according to need. Suitable binders include polyvinyl butyral, VAE, EVA, EAA, EEA, and polybutylmethacrylate.

The relative proportions of the materials in the curable mixture may be varied widely according to the desired balance of properties before and after curing. For example, in parts by weight % of mixture.

Widest Preferred Best

Polymer 30-90 40-60 50 Monomer 10-70 25-45 35 Peroxide 0.1-10 1 -5 1 .5 " CJRlAl

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Preferably, the mixture will be formulated to remain somewhat flexible after curing.

Other additives which may be used in known manner and proportions include fillers such as carbon black, (5% preferred), plasticisers such as 2-ethylhexyldi- pheylphosphate (15% preferred), and corrosion inhib¬ itors such as disodium hydrogen orthophosphate (2% preferred) .

In alternative forms of the invention, the cat- alyst may be included in the curable monomer/polymer mixture and the free radical generator provided separ¬ ately, but the aforementioned forms with the free- radical generator in the mixture are preferred for optimum curing in practice.

Some specific examples of the present invention will now be described by way of illustration.

Materials

ATM5 polyethylene glycol d imethacrylate , M .W.

330 , Ancomer. Santomer 210 polyethylene glycol dimethacrylate , M .W.

330 , Santomer. Sant i c i z er 1 4 1 2 -e thyl he xyl d iphenyl phos ph at e

Monsanto. Plexigum P24 polybutyl methacrylate , Rohm.

Example 1 Parts by wt 3. Plexigum P24 45 900 VAE 711 5 100 Santomer 210 30 600 Santicizer 141 15 300 tertiary butyl peroxy benzoate 2.5 50 ^^T f O

The premixed Plexigum P24 and VAE 711 (coarse powders) were added with hand stirring to the premixed Santomer 210, Santicizer 141 and tertiary butyl peroxy benzoate liquids, and stirred to break down any aglom- erates. The paste was poured into a 100mm X 150mm X 1.5mm mould on heat-shrinkable polyolefin/EVA sheet (30% free recovery) and a doctor blade passed across the mould to give a 1.5mm thick coating on the sheet. A sheet of siliconised paper was laid over the ad- hesive. After 1 hour the adhesive had 'gelled* and the siliconised paper could be peeled off. The adhesive was bonded to steel as follows using a primer solution of cobalt (II) acetylacetonate (2 pts) and N,N-dimethyl -p-toluidine (25 pts) in trichloroethane (50 pts) plus methyl ethyl ketone (50 pts).

A 2" diameter mild steel tube of 1/4" wall thick¬ ness was grit blasted then degreased with methyl ethyl ketone. The steel was then primed with the solution and the excess primer was wiped off and the solvents allowed to evaporate.

The adhesive-coated sheet was wrapped around the steel tube and held as a complete tube by means of an adhesive patch. The sheet was then recovered by heating with a gas torch, and the bond left 24 hrs at 40 ^β C. After the 24 hrs the sheet was cut open. The adhesive was found to have cured and formed a strong bond between the steel and the backing sheet.

Example 2

As Ex. 1 except that N-phenyl diethanolamine was used in place of N,N-d imethyl-p-toluidine in the primer. The results were similar.