ADHESIVE COMPOSITION

Abstract: An adhesive composition comprises a blend of: (a) at least one polyamide; and (b) at lest one acrylate-terminated prepolymer. The adhesive may be applied as a coating on a dimensionally recoverable, preferably heat shrinkable, sleeve for the environmental protection of substrate.

Description: ADHESIVE COMPOSITION The present invention relates to an adhesive composition, particularly to a hot-melt adhesive composition containing polyamide and being suitable for use in the cable accessories or pipeline industry, particularly for adhesion to non-flame-brushed polyethylene and/or lead. Environmental protection frequently has to be provided around substrates such as cables to protect them from contaminants such as moisture. An excellent way of providing such protection is to surround the substrate with an adhesive composition, conveniently applied as a coating on a dimensionally - recoverable, preferably heat-shrinkable sleeve. Such a sleeve may be positioned around the substrate and heated causing the adhesive composition to become molten or softened and causing the sleeve to shrink, driving the adhesive into bonding engagement with the substrate. The sleeve itself may of course provide environmental protection, and the quantity of adhesive required may be reduced by coating the sleeve at only its edges to provide a bond to the substrate. A specific instance where environmental protection is required is around a cable splice, particularly in a multi-core telecommunications cable, where missing cable sheath (removed in order to form the splice) has to be made good. This may be done by installing a liner around the splice, the liner preferably being shaped to provide a smooth transition between the bulky splice and the smaller diameter cable or cables that enter and leave it. The liner is then surrounded by a heat-recoverable sleeve that extends past each end of the liner down onto intact cable jacket. The sleeve is internally coated with a hot-melt adhesive composition and, when heated, it shrinks down and becomes bonded to each cable jacket and to the liner between them. In this way leak paths from the outside environment into the now-formed splice case are avoided, and the conductors of the cable protected from moisture etc. Such splice cases may be expected to provide an environmental barrier for the life-time of the cable, say at least twenty-five years. The cable, and splice case, may be subjected to temperature variations ranging from say -40°C in winter to say +70°C in the case of a cable exposed to direct sunlight, and the cable may be internally pressurized. As will be appreciated, these requirements of life-time, temperature and pressure put severe demands on the performance of the adhesive. The problem is made more difficult because for practical purposes, a given product may have to be suitable for many different environments, for example around buried cables in cold countries, and aerial cables in hot-countries. These performance requirements are themselves difficult to achieve at an acceptable price, but the difficulty can be seen to be particularly acute when one considers installation conditions. Installation is at present carried out by heating the sleeve and consequently the adhesive with an open-flame torch, although electrical heating may be used. It is clearly desirable that the amount of heat required to soften the adhesive be kept to a minimum. The weather may be cold, there may be little room in a manhole for vigorous heating, an electrical heater may have limited power, and the cable or the sleeve may be damaged by high temperatures etc. Thus, there is a maximum acceptable installation temperature. Also as noted above there is a wide temperature range over which the adhesive must retain its adhesive power. Unfortunately, the maximum allowable installation temperature is not very much higher than the minimum acceptable upper limit to the range of service temperatures. We have found therefore that our adhesive should have a sharp melt transition. In addition to a sharp melt transition, the adhesive should have a high peel strength to polyethylene, preferably both flame- brushed and non-flame-brushed, and to lead, it should have good low temperature impact strength, and low temperature flexibility, and it should have low viscosity at the installation temperature say at 160°C. Attempts have been made to formulate adhesive compositions that combine, for example, high impact strength and high peel strength over a wide temperature range. The following adhesive compositions based on polyamides may be noted. US 4018733 (Lopez, Glover and Lyons) discloses a hot-melt adhesive composition comprising a compatible mixture of: (a) an acidic polymer of ethylene having an acid number ranging from about 3 to about 80 selected from the group consisting of a terpolymer of ethylene, an ethylenically unsaturated mono-or di-carboxylic acid and a vinyl ester of Ci to Cβ aliphatic carboxylic acid and a copolymer of ethylene and a C2 to C20 aliphatic ester of a monoethylenically mono-or di- carboxylic acid wherein a portion of the carboxylic acid moieties of the said mono-or di-carboxylic acid component are not esterified, and (b) a polyamide having an amine number ranging from about 70 to about 400 with a tackifying agent wherein said tackifying agent is present in an amount ranging from about 3 to about 20 parts by weight per 100 parts of (a) and (b) together, components (a) and (b) being present in a relative proportion by weight ranging from about 40:60 to 60:40. EP 0040926 (Raychem) discloses a hot-melt adhesive composition comprising a polyamide based on one or more dimeric fatty acids and an ethylene/acrylic acid butyl acrylate terpolymer containing free carboxylic acid groups. The acid terpolymer is preferably present at an amount of less than 20% by weight based on the weight of the polyamide. GB 1563030 (Allied Chemical Corporation) discloses a hot- melt adhesive composition comprising a compatible admixture of: (a) 15 to 99 weight percent of a polyamide adhesive based component; and correspondingly (b) 85 to 1 weight percent of a normally solid, homogeneous copolymer component of ethylene and an unsaturated carboxylic acid having an acid number of at least 70 and a number average molecular weight between 500 and 5000, said copolymer containing at least a major portion of ethylene by weight, said percentages being by weight of (a) plus (b). We have recently forumulated an adhesive composition that can provide excellent peel strength to polyethylene, can have excellent low temperature impact strength and flexibility, and can be produced at reasonable cost. That new adhesive also can allow good installability due to low viscosity at installation temperatures. We have found that an ethylene copolymer of high melt flow index can be blended with suitable quantities of one or more polyamides to produce the desired properties. That adhesive, disclosed in EP 0326319 (Raychem), comprises a blend of (a) (at least one) polyamide; and (b) a copolymer of ethylene and an ester of an ethylenically unsaturated carboxylic acid, said copolymer having a melt-flow index of at least 40, preferably at least 70, expecially from 100-300, as measured according to ASTM D1238-70; the amount of the copolymer being at least 38%, preferably from 38- 126% based on the weight of the polyamide. We have now discovered an alternative adhesive formulation in which we employ an acrylate terminated pre- polymer, which we have found, can lead to improved low temperature properties and high temperature cohesive strength. Thus, the present invention provides an adhesive composition, more particularly a thermally reversible hot-melt adhesive, which comprises (a) at least one polyamide; and (b) at least one acrylate terminated pre-polymer. The acrylate terminated pre-polymer can be, for example, an acrylate terminated polyester , an acrylate terminated epoxy or an acrylate terminated bisphenol A derivative, but is preferably an acrylate terminated polyurethane. Preferably the acrylate terminated pre-polymer has a functionality of 2. The acrylate terminated pre-polymer may be aliphatic or aromatic and preferably has a molecular weight of from 3500 to 6500, more preferably from 4000 to 6000, especially about 5000. Examples of suitable acrylate terminated pre-polymers are those sold under the trade mark Ebecryl by UCB. Examples of suitable acrylate terminated polyurethanes include Ebecryl 210, (aromatic) and Ebecryl 230 (aliphatic); an example of an acrylate terminated polyester is Ebecryl 810; examples of acrylate terminated epoxies are Ebecryl 6000 and Ebecryl 608; and an example of an acrylate terminated bisphenol A derivative is Ebecryl 150. The invention also provides a method of making a hot-melt adhesive in which a reactive pre-polymer is melt mixed with a polyamide, causing reaction and modification of the polyamide, thereby to improve low temperature flexibility, high temperature cohesive strength, and impact strength, reaction being indicated by a lowering of the amine number of the polyamide. The pre-polymer may be used together with the copolymer of ethylene and ester referred to above and described in EP 0326319. The composition may additionally comprise other components for example for controlling viscosity or flow temperatures or adhesive strength. A preferred further component comprises a polyethylene acrylic acid copolymer, (which can help control viscosity or flow temperature), which is preferably present up to 57% based on the weight of the polyamide, and is preferably less than 20% based on the weight of the composition, more preferably less than 15%, especially less than 10%, say 3-7% particularly when low acidity is desired. Other components may comprise Versamid 100 (trade mark for polyamide of Schering), tackifiers, such as a terpene phenol, for example that known by the trade mark SP553 (Schenetady Corp.), polyisobutylene and antioxidants such as that known by the trade mark Irganox 1010 (Ciba Geigy). In some circumstances it is preferably that the composition contains substantially no vinyl acetate or vinyl acetate polymers or copolymers. Cross-linking of the composition can be avoided by a proper choice of the amine number of the polyamide, and the quantity and nature of the acrylate terminated prepolymer. A preferred adhesive composition comprises: (a) from 40-70, preferably 50-65 parts by weight of one or more polyamides; (b) from 10-30, preferably 15-25, especially 15-20 parts by weight of one or more copolymers of ethylene and an ester, particularly polyethylene ethylacrylate and/or polyethylene butyl aery late; (c) from 5-20, preferably 5-15, especially 5-10 parts by weight of polyethylene acrylic acid copolymer; and (d) a polyurethane acrylate, preferably up to 10, especially up to 5, particularly 1-3 parts by weight. The or each polyamide may have a ring and ball softening point according to ASTM E28 of from 80-120°C, more preferably from 90-110°C, a molecular weight of from 2000-10000, and a viscosity at 210°C according to ASTM D3236 of 2-8, preferably 3- 5 Pa.S. and/or a viscosity at 160° of 30-50 Pa.S., and an amine number from 5-400. For some uses, however, polyamides with a low amine number, say from 5-12, for example about 7, will be preferred. Suitable polyamides include those known by the trade marks Macromelt 6735 (Henkel), Macromelt 6766 (IJenkel) Macromelt 6301 (Henkel), TPX 22-143 (Henkel), TPX 20-033 (Henkel) and Macromelt 6768 (Henkel). In, .general, a polyamide (or blend of polyamides) may be chosen that has the desired physical and chemical properties (for example melt flow index, softening point, viscosity, and adhesive strength) substantially similar to those of the quoted examples. Two (or more) polyamides may be chosen particularly where low temperature flexibility is required, especially in combination with low cost. The copolymer of ethylene and an ester may contain two or more comonomers. Preferred copolymers for some uses have a ring and ball softening point according to ASTM E28 of 80-125°C, especially 95-120°C. Examples of preferred copolymers and terpolymers include: polyethylene ethylacrylate, such as those known by the trade marks Lotader 8600 (formerly known as Lotader 8200) (Charbonage de France), Lotader 7500 (Charbonage de France), EA 89821 (USI) and Alathon 704 (DuPont); and polyethylene butylacrylate, such as that known by the trade marks Lotader HX 8280 (Charbonage de France) and Lucalene A2920-M (BASF). As with the polyamide, other copolymers can be used having similar physical and chemical properties to those quoted. Where a polyethylene acrylic acid copolymer is used, we prefer that known by the trade mark Alathon 5120 (DuPont) or AC5120 (Allied Chemical Corporation), now known as AC143 or equivalents, although polyisobutylene such as that known by the BASF trade mark Opanol B-12 may be used additionally or in its place. We have identified a problem that may occur when using some prior art adhesives, and that may be solved with the present invention. Adhesion may be poor to certain materials, particularly metals, such as that used for lead cables in Belgium. We have noticed that the lead used for Belgian cables is substantially pure and therefore soft. As a result of this softness, and as a result of a disparity between the thermal coefficient of expansion of adhesives and of lead, thermal cycling may cause cohesive failure of the lead : a very thin surface layer of the lead may peel away from the bulk. The problem is less likely to occur with harder lead compositions, for example those containing impurities or additives such as antimony, copper, silver or tin. The skilled man will be able to determine where there is a risk that this problem will occur, but typically at risk are lead compositions having a melting point as determined by differential scanning calorimetry at 8 degrees centigrade per minute of at least 322 degrees centigrade. An alternative indication (which applies to lead and to other metals and other materials) is hardness : a Vickers hardness of less than 5.5, preferably less than 5.0 microns may lead to poor bonding, the measurement being made on Leitz Miniload equipment using a 100 gm core. Recognition of this possible failure mode has led us to reduce the acidity of our adhesive composition and thereby reduce stress on cycling at the adhesive/lead interface. This may be done by, for example, reducing the acidity of say AC 5120, or by replacing or reducing the amount of the AC 5120 itself. In one embodiment AC 5120 may be at least partially replaced by SP 553 (a trade mark of Schenectady, France, for a tackifier), and/or by AC 540 (a trade mark for a polyethylene acrylic acid copolymer of Allied Chemical Corporation). AC 540 has a lower concentration of acid functionality than AC 5120. Peel strengths at room temperature of at least 100 N/25mm, generally at least 150 N/25mm to Belgian lead can be achieved after 10 temperature cycles from -40 degrees centigrade to +60 degrees centigrade. The primary components selected, and their relative amounts, and any further components such as viscosity or flow modifiers are preferably such as to achieve a peel strength to flame-brushed polyethylene of at least 300 N/25mm,_ and to non- flame brushed polyethylene and preferably also to lead of at least 100 N/25mm at room temperature (Raychem published test QAPK027), a ring and a ball softening temperature of 90-110°C (test ASTM E28), a viscosity at 160°C of 40-110 Pa.S (test ASTM D3236), a low temperature mandrel flexibility of -30 β C or lower, and an impact brittleness of -30°C or lower, and an impact brittleness of -30° or lower (test ISO 974). The adhesive composition may be provided with, preferably as a coating on, a dimensionally-recoverable, preferably heat- shrinkable article. The article may comprise a sleeve, which may be tubular or of the wrap-around type, such as that disclosed in UK Patent 1155470 (Raychem). We particularly prefer that the sleeve or other recoverable article comprise a recoverable composite material recoverable by virtue of a recoverable fibre component thereof. The recoverable fibre may be part of a recoverable fabric such as one that comprises a weave of recoverable fibres in one direction and dimensionally stable fibres in another direction. We prefer that the recoverable fibres comprise polyethylene and that the stable fibres comprise glass. The composite material may comprise recoverable fibres and a matrix material, such as polyethylene, by means of which the composite is rendered substantially impermeable. The matrix material is preferably such that the adhesive composition is able to bond thereto. Where a branched cable splice is to be environmentally sealed by means of a sleeve, a branch-off clip may be applied to an end of the sleeve to hold together circumferentially-spaced portions of the sleeve between the branching cables. The use of such a clip to bring together such portions of a recoverable sleeve is disclosed in GB 1604981. Such a clip may have three legs, the outer two lying outside the sleeve, and an inner lying within the sleeve, generally between the branching cables. The branch-off seal formed using such a clip may be enhanced if an inner leg of the clip comprises an adhesive, and the adhesive of the invention may be used for such purpose. The adhesive may provide substantially the whole of the inner leg, or may provide a coating over a heat-conducting or other support. The adhesive of the invention may be supplied separately from any sleeve instead of or in addition to the sleeve coating as mentioned above. For example, such adhesive may be supplied in strip form for wrapping around a cable prior to installation of a recoverable sleeve or other cable enclosure. The invention is further illustrated by the following examples. Example 1 An adhesive composition is prepared by mixing Macromelt 6301 with various Ebecryl resins. The resulting adhesive composition has better low temperature properties and improved high temperature cohesive strength compared to the unmodified Macromelt 6301 as demonstrated in Table 1 below: TABLE 1 Example 2 An adhesive composition is made by mixing in a blend mixer the following materials: Polyamide (TPX 22-143, Henkel) 32 parts by weight Polyamide (Macromelt 6768, Henkel) 30 Polyethylene Butyl acrylate 1 8 (Lotader 8600 B, Orkem) Polyethylene acrylic acid (AC 143 Allied Chemicals) Polyisobutylene (Opanol B-12, BASF) 8 Tackifier (SP553, Schenectady) 4 Polyurethane Acrylate 2 (Ebecryl 230,UCB) Antioxidant (Irganox) The resulting adhesive composition is then laminated onto a heat-recoverable composite comprising a weave of recoverable high density polyethylene in the weft and glass fibres in the warp having a matrix material of low density polyethylene. The lamination is carried out to a thickness of 0.4mm using a belt laminator. A wrap-around sleeve is formed from the resulting coated composite by forming at edge regions thereof closure members that can be held together by a C-shaped channel, as disclosed in EP 0116392 (Raychem). The sleeve is then heat-shrunk around a polyethylene cable splice of 1 in 3 out configuration, using a branch-off clip between the branching cables. The size of the sleeve is that known by Raychem' s trade mark VASM 4/6. The sleeve was tested for leaks on a pressure/temperature cycle at 40 KPa from -40°C to +60°C at 3 cycles per day. Excellent performance is noted. Adhesive compositions prepared according to this example have a viscosity at 200°C of 15-22 Pa.S and at 160°C of 82-85 Pa.S, and a ring and ball softening point of 99-100°C. Viscosity is determined according to ASTM D-3236-1978, using a Brookfield Model HBT, Spindle SC-27 at 160°C at a shear rate of 5 rpm. The softening point is determined by the ring and ball method according to ASTM E28-1977. The sample preparation being by the pour method, at a heat-up speed of 5°C/minute. Peel strengths at room temperature to flame-brushed polyethylene range from 351-395 N/25mm, and to non-flame- brushed polyethylene rang from 101-126 N/25mm. Peel strengths to flame-brushed polyethylene at 60°C range from 81- 82 N/25mm. - 13 - Excellent peel strengths to lead cable are also noted. Before cycling a value of 146 N/25mm is obtained, and after 10 cycles from -40°C to +60°C a value of 152 N/25mm is obtained, and after 30 such cycles, 149 N/25mm. Low temperature impact strength tests give a value of -39°C Similar results are achieved by using 62 parts by weight of TPX22-143, instead of the mixture of two polyamides indicated above. Example 3 An adhesive composition is made by mixing in a blend mixer the following materials: MM 6768 39 parts by weight MM 6301 39 Irganox 1 Ebecryl 210 4 "• * Lucalene 2920 16.5 Hycar 4054 0.5 The composition has the following physical properties. Viscosity (PaS) 160°C 150°C Ring and Ball °C 99 Peel to PE (N/25mm) Room temperature flame-brushed 330 Non flame-brushed 167 Peel at 60°C to Flame-brushed PE (N/25mm) 140 Example 4 An adhesive composition is made by mixing in a blend mixer the following materials: MM 6768 42.5 parts by weight MM 6301 3 7 MM 6238 5 Irganox 1010 1 Ebecryl 210 2.5 Hycar 4054 0.5 Lucalene 2920 10.5 Lotader 8200 1 The composition has the following physical properties: Example 5 An adhesive composition is made by mixing in a blend mixer the following materials: MM 6768 69.5 parts by weight MM 6301 15.0 Irganox 1010 1.5 Hycar 4054 0.5 Lucalene 2920 9.5 Ebecryl 210 3 The composition has the following physical properties: Macromelt 6301 polyamide having an amine equivalent number of 103 is melt-blended in a blend mixer with various quantities of an aromatic polyurethane acrylate, (Ebecryl 210 molecular weight 1500, functionality 2) and an aliphatic polyurethane acrylate (ebecryl 230, molecular weight 5000, functionality 2). The amine equivalent number of each formulation is measured and the results given in Table 2 below. TABLE 2 These results show that melt-blending the polyamide and the polyurethane acrylate produces a reaction, as evidenced by the decrease in the amine equivalent number of the blend. For the avoidance of doubt, it is here stated that the invention provides an adhesive composition, and an article particularly a recoverable article having such a composition on or at a surface thereof, that exhibits the properties mentioned herein as desirable. Any of the components mentioned herein may be selected to achieve these properties, particularly the selection of a polyurethane acrylate together with a polyamide, a modification to the polyamide possibly to reduce amine number possibly occurring during melt mixing and inclusion of other components such as acid copolymers, viscosity or melt flow temperature modifiers, rubbers and antioxidants, etc.

Claims: CLAIMS 1. An adhesive composition comprising a blend of: (a) at least one polyamide; and . (b) at least one acrylate terminated pre-polymer. 2. An adhesive composition according to claim 1, in which the acrylate terminated pre-polymer is a polyurethane acrylate. 3. An adhesive composition according to claim 1 or 2, having up to 10 parts by weight of the acrylate terminated pre-polymer and from 40-70 parts by weight of the polyamide. 4. An adhesive composition according to any preceding claim, which additionally comprises a polyethylene acrylic acid copolymer. 5. An adhesive composition according to claim 4, in which the amount of the polyethylene acrylic acid copolymer is less than 10% based on the weight of the polyamide. 6. An adhesive composition according to any preceding claim, which additionally comprises a copolymer of ethylene and an ester of an ethylenically unsaturated carboxylic acid. 7. An adhesive composition according to claim 6, comprising: (a) from 40-70 parts by weight of polyamide; (b) from 10-30 parts by weight of copolymer of ethylene and an ester; (c) from 5-20 parts by weight of polyethylene acrylic acid copolymer; and (d) up to 10 parts by weight of a polyurethane acrylate. 8. An adhesive composition according to claim 6, in which said copolymer of ethylene and an ester comprises polyethylene ethyacrylate or polyethylene butylacrylate. 9. An adhesive composition according to any preceding claim in which polyamide (a) comprises a blend of two or more different polyamides. 10. An adhesive composition according to any preceding claim which comprises: (a) from 50-65 parts by weight of polyamide; (b) from 15-20 parts by weight of polyethylene butyl acrylate copolymer; (c) from 5-10 parts by weight of polyethylene acrylic acid copolymer; and (d) from 1-3 parts by weight of a polyurethane diacrylate having a molecular weight from 4000-6000. 1 1. A heat-shrinkable article having an adhesive composition according to any preceding claim on or at a surface thereof. 12. An article according to claim 11, being a recoverable composite material recoverable by virtue of a recoverable fibre component thereof. 13. An article according to claim 12, in which said composite material comprises recoverable fibres and a polyethylene matrix material the adhesive composition being bonded to the matrix. 14. An article according to any of claims 11-13 in the form of a tubular or wraparound sleeve. 15. A method of environmentally protecting a cable or a pipe which comprises (a) providing an adhesive composition according to any of claims 1-10; (b) heating the composition; (c) allowing or causing the composition to flow around a surface of the cable or pipe; (d) allowing or causing the composition to cool. 16. A method according to claim 15, in which the composition is provided as part of a heat-recoverable article. 17. A method of making a hot-melt adhesive which comprises melt mixing a reactive pre-polymer with a polyamide, causing reaction and modification of the polyamide, as evidenced by a lowering of the amine equivalent number of the polyamide, and an increase in low temperature flexibility and impact strength. Jl * * * * * * * * *

Inventors: OVERBERGH NOEL MARCEL MICHIEL, OVERBERGH, NOEL, MARCEL, MICHIEL
Applicants: RAYCHEM LTD, RAYCHEM SA NV, N.V. RAYCHEM S.A., RAYCHEM LIMITED
Application year: 1990
Main classification: H01B7
IPC codes: B29C61, B29C63, C08F290, C08F299, C08J5, C09D177, C09J123, C09J155, C09J157, C09J177, H01B17, H02G15