HEAT CURABLE COMPOSITION

Abstract: A curable powder composition comprising two or more separate powdered reactive components is provided with a thin coating of binder on the powder particles under conditions which leave the particles separate from one another. This com­ position can be rendered cohesive, e.g. by compression, to form self-supporting but surprisingly flexible sheets or other ar­ ticles of the curable adhesive, while retaining the particles in discrete form within the sheet to retain the desirable balance of storage life against curing performance derived from the par­ ticulate form of the reactants.

Description: This invention relates to heat-curable compositions of the kind which cure by reaction of a first reactive component with a second reactive component when heated to the curing temperature. Such compositions may be useful, for example, as curable adhesives. Ready-mixed heat-curable adhesives which cure by reaction of two components generally suffer from the disadvantage that either their shelf life is good but their eventual cure rate is slow as a result of measures taken to prevent premature curing, or their cure is good but their shelf life inadequate due to premature curing. British Patent Specification No. 2104800 describes an ingenious solution to this problem, wherein a mixture of separate powders of the reactive components, which mixture has excellent shelf life, provides surprisingly good cure when a heat-shrinkable polymer sleeve is shrunk onto the mixture at or above the curing temperature. However, the coherent uncured adhesive layers which may be formed for application to the sleeve by compressing the mixture, while retaining its advantageous balance of shelf life and cure, tend to be undesirably brittle and are easily cracked, broken and lost in use. The described addition of unspecified amounts of pressure - sensitive adhesive or mastic of course inhibits the cure, while the use of more flexible reactants tends to result in premature cure. The present invention improves the flexibility and handling character of such adhesive layers in the uncured state while maintaining an advantageous balance of shelf life and curing properties. EP-A-0117738 does not relate to mixtures of two reactive particles at all. It is concerned with sheets or tapes having extruded regions containing a curable material adjacent to extruded regions containing a curing agent, arranged for example in stripes or checks. Such a product thus has a very different structure, produces a very different level of cure, and is used for very different purposes, from the intimately mixed fine particles of the present invention. The invention provides a heat-curable composition comprising a substantially incohesive mixture of substantially solid particles which mixture includes substantially solid particles of a first reactive component and separate substantially solid particles of at least a second reactive component capable of reacting with the first component to effect curing when the composition is heated to the curing temperature, wherein at least a majority of the particles carry individual portions of compatible (as hereinafter defined) binder, the total weight of binder being within the range from 0.1% to 30% of the total weight of the composition, provided that, if the viscosity of the binder per se is not more than 20,000 centipoises at 25°C then (a) the combined weight of binder is not more than 15% of the composition and (b) at least 10% by weight of the total discrete particles have a deformability (as hereinafter defined) of at least 50%, and wherein the binder is capable of binding the composition together to form a self-supporting room-temperature-flexible article when the composition is compressed under conditions which allow at least the first reactive component and the second reactive component to remain in particulate form. This invention is thus based upon the unexpected discovery that pre-coating of the reactant particles with less than 30% or 25%, preferably not more than 20%, more preferably not more than 10%, and most preferably not more than 5%, binder, by weight, results in a composition which can be rendered cohesive to form a self-supporting flexible and robust article, such as a sheet strip, rod, tube, or other form of the latent curing adhesive with an advantageous balance of properties as aforesaid. The microstructure of such adhesives after processing but before cure is predominantly heterogeneouS, the particles retain their individual identity and do not substantially coalesce. The composition may be rendered cohesive to form the desired article by various means, for example by compression at a temperature below the softening point of the reactive components, preferably in the substantial absence of any solvent for the binder or the reactive components. Forming techniques such as rolling, extrusion, or moulding, at a temperature below the melting point of the reactive components, may be used. Preferably the reactive component particles are bound together substantially only by the said binder. Other useful adhesive articles may be formed from the powder composition, e.g. substantially rigid rings or discs. The binder may be applied to a mixture of the reactive powders, or separately to the respective powders which are subsequently mixed together. The article is obtained by two ambient temperature processing steps. Firstly the binder is distributed around the powders in a high speed, low shear mixer. Secondly, the powder/binder mixture is formed into a sheet or other shape (e.g. adhesive ring) by cold pressing or cold milling. It is evident that much lower pressures are needed to form this sheet than in previously described techniques thus allowing greater range of sheet thickness and more varied preform shapes to be produced. Preferably the binder per se is flowable under substantially solvent-free processing conditions at temperatures which do not cause prereaction of powders and which do not significantly render flowable any reactive components particles, and has been spread over at least some of the individual particle surfaces under such conditions, preferably by application to the mixture of the particles of the first and second reactive components. Thus, the invention provides a method of making the incohesive mixture of binder-coated particles, comprising dispersing the binder into a mass of particles of the first reactive component and/or particles of the second reactive component under conditions under which the reactive component particles are substantially non-flowable and the binder is flowable to coat the individual particle surfaces. Preferably, the binder is dispersed into the particles under substantially solvent-free conditions. It may also be preferred that the binder is applied to the particles of the first reactive component separately from the particles of the second component and/or is applied to the particles of the second reactive component separately from the particles of the first reactive component, and the respective particles of the first and second reactive components are subsequently mixed together. "Substantially solid" particles, as referred to herein means particles which are substantially non-flowable in the composition when stored at room temperature and excludes liquid-filled microcapsules. "Compatible binder" means binder material which (1) does not significantly cure the composition or render flowable any reactive component thereof at storage temperatures below the intended maximum for the composition, and (2) does not significantly inhibit curing at the intended curing temperature. The "separate" particles, e.g. of the reactive components, are preferably separate in the sense that the particles of one kind are freely movable relative to the particles of the other kind(s) in the mixture, for example when the mixture is formed by mixing together two separate powders of the first and second reactants respectively. An important aspect of the present invention thus relates to the use of low levels of specified binders which are processed without the use of solvents to give latent curing adhesive materials in the form of self-supporting flexible sheets or pre-moulded shapes or extruded profiles. The use of binders allows processing of materials having good flexibility and a relatively high resistance to mechanical abuse in the uncured state whilst maintaining the advantageous balance of shelf life and curing properties. Preferably the reactive components and binder comprise at least 70%, more preferably at least 75%, most preferably at least 80%, by weight, of the composition. Preferably the reactive components are substantially non flowable under storage and processing temperatures. The powders are treated by mixing with a binder using a high speed low shear mixer. After treatment the powders carry individual portions of the binder, the total weight of the binder being within the range of from 0.1% to 30% of the total weight of the composition. Preferably, the total weight of the binder is not more than 15%, more preferably not more than 10%, most preferably not more than 5%, of the composition, and is not less than 1%, more preferably not less than 2%, of the composition. The nature and level of the binder used are dependent upon the deformability of at least one of the powdered components. If the viscosity of the binder is not more than 20000 centipoise, preferably not more than 15,000 centipoise, at 25 o C then (a) the combined weight of binder is preferably not more than 15%, preferably not more than 10%, most preferably not more than 5% of the composition and (b) at least 10%, preferably at least 15%, more preferably at least 25%, by weight, of the powders have a deformability (as hereinafter defined) of at least 50%. When the binder (1) does not significantly cure the composition or render flowable any reactive components at the said storage temperatures, and (2) does not significantly inhibit curing at the intended cure temperature, then a self-supporting flexible curable sheet may be formed when the composition is compressed under conditions which allow the binder on the particle surfaces to consolidate the sheet but retain the first reactive component and the second reactive component in particulate form although deformable particles may be changed to an ellipsoid or other shape under said conditions. If the viscosity of the binder is greater than 15,000, preferably greater than 20,000 or 35,000, centipoise at 25 o C then similar self-supporting flexible articles may be formed with substantially no particles of more than 50% deformability providing the combined weight of the binder is not more than 25% and preferably more than 15% of the composition, although less than 15% binder may be used. DETAILED DESCRIPTION OF THE INVENTION Preferably the binder is flowable under substantially solvent free processing conditions which do not render flowable any reactive component particles. The binder is spread over at least some of the individual particle surfaces using a high speed, low shear mixer. At this stage the adhesive remains as a free flowing powder and is formed into its given shape by subsequent ambient temperature processing. For example, by passing the powder-binder mix through a two-roll mill a robust, flexible self-supporting sheet is formed. By compressing the mix on a high pressure tablet press, pre-moulded shapes can be readily formed. By passing the mixture through an extruder barrel various extrudate profiles, including tubes, can be readily formed. The cohesive article may, for example, be flexible preferably at room temperature to the extent that it can be wrapped without clearly observable cracking around a circular body of diameter 100 times, preferably 50 times, more preferably 25 times, the maximum thickness of the article in the sense which would lie perpendicular to the circular body surface when the article is wrapped once around. In some cases, the article can be wrapped without clearly observable cracking around a circular body of diameter 10 times, preferably 5 times, the maximum thickness of the article in the sense which would lie perpendicular to the circular body surface when the article is wrapped once round it. Preferably such an article has a thickness of from 0.2 to 5, preferably 1 to 3, millimetres in the said perpendicular sense. BINDER MATERIALS A wide range of materials are suitable for use as binders providing they are (1) substantially liquid at processing temperatures, preferably having a viscosity at 25° of not more than 600,000 centipoises, more preferably not more than 200,000 centipoises, most preferably not more than 100,000 centipoises, and especially not more than 20,000 or 15,000 centipoises; (2) remain substantially non-volatile under storage and processing conditions and (3) do not solubilise the reactive powder components of the adhesive. Liquid polymers and oligomers with viscosities from 300 to 1,000,000 centipoise, as measured at 25°C, can be used as binders. These liquids can be broadly classified as polyalkenyl anhydrides, copolymers of alkenes and ethylenically unsaturated acids or nitriles, polyglycols, polyols, polysiloxanes, polybutadienes, polybutadiene/acrylonitrile copolymers, polyisobutenes, polyesters, aliphatic (preferably 14 or more carbon atoms) hydrocarbons, which are substantially non-volatile at the relevant storage temperatures, acrylic adipates, arylphosphates, orthophosphates, , palmitates, sebacates, sterates, acrylated oligomers and methacrylated oligomers. Other examples of binder materials will be apparent to those familiar with liquid polymers. Of use for powdered formulations where at least 10% of the powders are deformable are binders of viscosity less than 15,000 centipoise especially not more than 10,000 centipoise, preferably not more than 5000 centipoises, more preferably not more than 1500 centipoises, at 25°C. Of particular use are polypropylene glycols and polybutadienes. Of use as binders for powder formulations containing no deformable powders are liquid polymers with viscosities greater than 10,000 centipoise at 25°C. Particularly useful are the butadiene acrylonitrile copolymers and the polybutadienes. Binders which are substantially solid at room temperature but become liquid or highly deformable under conditions used to apply them to the particles (eg. the binder is flowable at 250°C) are also useful; aliphatic hydrocarbons such as nonodecane can be used as a binder above its melt point (33 o C) but below the softening point of the reactive resin components. Soft deformable solid polymeric resin powders may also be used as binders providing the processing conditions cause such powders to coalesce and bind substantially solid powders which are not deformed during processing. The binder may be capable of reacting with either or both of the reactive components when the composition is heated to the curing temperature. The composition may include particles of a substantially non-reactive component having a deformability of at least 50% to assist in producing a coherent article. POWDERED MATERIALS The powders which can be used are solid resins at room temperature and remain solid during processing and storage. The powders can be substantially solid particles of hard brittle polymers or oligomers, having glass transition temperatures above processing and storage conditions, or they can be substantially solid deformable particles of polymers having a melting point above the storage and the processing temperature but having a glass transition temperature below this temperature. The deformable substantially solid particles having at least 50% deformability are defined as those which in disc form, after compression by a force of 2000 Newtons per square millimetres, remain deformed by at least 1.5 times their original diameter. The deformable powder may not necessarily be a reactive powder and may be inert towards the other powders. Powders which are usable in this invention can be classified as powders from reactive resins (the "first reactive component"), powders of suitable curing agents (the "second reactive component") and powders which are substantially inert towards the curing components. REACTIVE POWDERS The substantially solid reactive particles of the "first reactive component" can be from epoxide resins, phenolic resins and acrylic resins. Solid epoxide resins of use in the invention include:- diglycidyl ethers of bisphenol A, diglycidyl ethers of bisphenol F, polyglycidyl ethers of ortho-cresol-formaldehyde novalacs, polyglycidyl ethers of phenol formaldehyde novalacs, triglycidyisocyanurates, copolymers of bisphenyl A and epichlorolydrin (Phenoxy) and tetrafunctional polyglycidyl ethers of tetraphenylene ethane. Other usable reactive powders are acrylic resins, such as polybutyl methacrylate and acrylic acid adducts of solid diglycidyl ethers of bisphenyl A, and phenolic resins. CURING AGENT POWDERS The substantially solid particles of the curing agent "second reactive component" for the aforementioned resins are predominantly amine functional resins which may be brittle or deformable. Other resins such as anhydride resins and carboxylic functional resins can be used as curing agents. The solid brittle resins include crystalline materials such as dicyandiamide, imidazoles and substituted imidazoles, cationic salts and amine adducts of solid diglycidyl ethers of bisphenol A. The deformable curing agents resins are predominantly polyamide resins such as the dimer acid fatty polyamides. Of particular interest are deformable resins having amine valves greater than 10mg KOH per gram of resin. Blends of such low reactivity polyamides and liquid high reactivity polyamides or other amine functional resins can be prepared to achieve these properties. Thus, the second reactive component may for example be a thermoplastic polyamide , a thermoplastic copolymer of an alkene with an ethylenically unsaturated ester of an alkanol and a carboxylic acid, or a thermoplastic copolymer of an alkene with an ethylenically unsaturated acid or nitrile, or an amide adduct of a solid epoxy resin. INERT POWDERS Inert resins having little or no reactivity towards the reactive powders and their curing agents can be used to enhance processing capabilities, improve the cured resins properties and to reduce costs. Useful in this respect are powdered rubbers such as solid acrylonitrile/ polybutadine copolymers, powders of vinyl acetate ethylene copolymers and powders of chlorinated polyethylene. The invention includes the incohesive compositions, or preferably the self-supporting articles made therefrom, when carried on a dimensionally heat-recoverable article so as to lie, in use, between the recoverable article and an object about which the article is to be recovered in use. The invention also naturally includes the compositions and articles described above which have been cured. The following examples illustrate the invention, all parts expressed therein being by weight. EXAMPLE 1 Example 1 illustrates the use of a liquid binder to form a self supporting cohesive sheet, 20 parts of a cryogenically ground dimerised fatty acid reactive polyamide sold under the trade name Henkel Macromelt 6071, 50 parts of a powdered epoxy resin of the diglycidyl ether of bisphenyl A type sold under the trade name DOW DER 662 and 20 parts of a powdered ethylene diamine adduct of the diglycidyl ether of bisphenyl A sold under the trade name Anchor EDA 870 were placed in a high speed low shear mixer with 7 parts of a polypropylene binder sold under the trade name DOW PPG 4000. The ingredients were mixed at room temperature using a drive speed of 2,000 RPM for 2 minutes on a Papenmier high speed low shear 5kg mixer. The mixture was then taken and passed between the nip rolls at room temperature on a Carter 45cm (18 inch) 2 roll mill. The nip roll gap on the mill can be altered to control the final thickness of the adhesive material. Sheets of such adhesive were used to form lap shear bonds and cured for 30 minutes 120°C to give lap shear strengths of 2.8 MPa (400 psi) at 80°C. This sheet adhesive was laminated to a heat-shrinkable backing and shrunk around a constant temperature oil pipe of 5 cm (2 inches) Diameter, running at 80°C. After 24 hours the pipe was subjected to ASTM G42 cathodic disbonding test at 80°C and examined after 30 days under such conditions, revealing only a 10 millimetres disbondment radius. Examples 2 to 9, as shown in table A, illustrate the use of binders under 5,000 centipoise to form flexible self supporting sheets demonstrating the effects of deformable polymer content and deformability on ease of processing and sheet flexibility. Examples 10 to 22, as shown in table B, ilustrates the effects on binder viscosities on formulations containing 50 parts deformable powder and 0 parts deformable powder. Examples 23 to 34, as shown in table C, illustrates the ability to make self supporting adhesive sheets with a wide range of reactive resins and cure agents. Examples 35 to 37, as shown in table D, illustrate the use of inert resins for sheet manufacture. Examples 38 to 42, as shown in table E, illustate formulation which can be used to prepare preforms such as adhesive rings. These formulations were formed into adhesive rings of 9 mm external diameter and 6 mm internal diameter, thickness of 5 mm, on a Manesty F3 tabletting machine. Under the high compression forces used during processing the solid binders behave as a liquid and bind the hard powders of epoxy resin and cure agent together to give a more robust and handleable ring. For example formulation 42 (no binder) shatters at a drop height of 200 mm, where as formulation 40 (binder) does not break at a drop height of 600 mm. Example 41 was cured for 1 hour at 120°C to give a lap shear strength of greater than 7 MPa (1,000 psi) at 100°C. Example 40 was cured under a heat-shrink article on a 6 mm diameter pipe coupling to give a pressure retention of 21 MPa (3,000 psi) at 150°C.

Claims: A heat-curable composition comprising a substantially incohesive mixture of substantially solid particles which mixture includes substantially solid particles of a first reactive component and separate substantially solid particles of at least a second reactive component capable of reacting with the first component to effect curing when the composition is heated to the curing temperature, wherein at least a majority of the particles carry individual portions of compatible (as hereinbefore defined) binder, the total weight of binder being within the range from 0.1% to 30% of the total weight of the composition, provided that, if the viscosity of the binder per se is not more than 20,000 centipoises at 25°C then (a) the combined weight of binder is not more than 15% of the composition and (b) at least 10% by weight of the total discrete particles have a deformability (as hereinbefore defined) of at least 50%, and wherein the binder is capable of binding the composition together to form a self-supporting room-temperature-flexible article when the composition is compressed under conditions which allow at least the first reactive component and the second reactive component to remain in particulate form. A composition according to claim 1, wherein the binder per se is flowable under substantially solvent-free processing conditions which do not significantly render flowable any reactive component particles. A composition according to claim 1, wherein the total weight of binder is not more than 15%, preferably not more than 10% of the composition, more preferably not more than 5% of the composition, and is not less than 1%, preferably not less than 2% of the composition. A composition according to claim 1, wherein the binder is flowable at 250°C. A composition according to claim 1, wherein the viscosity of the binder per se is not more than 20000, preferably not more than 15000, centipoises at 25°C. A composition according to claim 5, wherein the viscosity of the binder per se is not more than 10,000 centipoises, preferably not more than 5,000 centipoises, more preferably not more than 1,500 centipoises, at 25°C. A composition according to claim 1, wherein the viscosity of the binder per se is greater than 20,000 centipoises at 25°C. A composition according to claim 7, wherein the viscosity of the binder per se is greater than 35,000 centipoises at 25°C. A composition according to claim 7 or 8, wherein the total weight of binder is more than 15% and not more than 25% of the composition. A composition according to claim 7 or 8, comprising substantially no particles of more than 50% deformability. A composition according to claim 1, wherein the first reactive component is a thermoplastic epoxy compound. A composition according to claim 11, wherein the second reactive component is a thermoplastic polyamide, a thermoplastic copolymer of an alkene with an ethylenically unsaturated ester of an alkanol and a carboxylic acid, or a thermoplastic copolymer of an alkene with an ethylenically unsaturated acid or nitrile. A composition according to claim 1, wherein the second reactive component is an amide adduct of a solid epoxy resin. A self-supporting heat-curable adhesive article comprising a composition according to claim 1, which has been rendered cohesive with at least the first reactive component and the second reactive component remaining in particulate form in the article. An article according to claim 14, wherein the reactive component particles are bound together substantially only by the said binder. An article according to claim 14 or 15 wherein at least some of the particles, preferably reactive component particles, have a deformability of at least 50% and been distorted by the process of rendering the composition cohesive. An article according to claim 14, which is flexible preferably at room temperature to the extent that it can be wrapped without clearly observable cracking around a circular body of diameter 100 times, preferably 50 times, more preferably 25 times, the maximum thickness of the article in the sense which would lie perpendicular to the circular body surface when the article is wrapped once around it. An article according to claim 17, which can be wrapped without clearly observable cracking around a circular body of diameter 10 times, preferably 5 times, the maximum thickness of the article in the sense which would lie perpendicular to the circular body surface when the article is wrapped once around it. An article according to claim 17 or 18 having a substantially uniform cross-section in the sense which would lie parallel to the circular body surface when the article is wrapped around it and having a thickness of from 0.2 to 5, preferably 1 to 3, millimetres in the said perpendicular sense. A method of manufacturing a composition according to claim 1, comprising dispersing the binder into a mass of particles of the first reactive component, and/or particles of the second reactive component under conditions under which the reactive component particles are substantially non-flowable and the binder is flowable to coat the individual particle surfaces. A method according to claim 20, wherein the binder is dispersed into the particles under substantially solvent-free conditions. A method according to claim 20, wherein the binder is applied to the particles of the first reactive component separately from the particles of the second component and/or is applied to the particles of the second reactive component separately from the particles of the first reactive component, and the respective particles of the first and second reactive components are subsequently mixed together. A method of manufacturing an article according to claim 14 comprising compressing the composition at a temperature below the softening point of the reactive components to render it cohesive. A method according to claim 23, wherein the composition is compressed to render it cohesive in the substantial absence of solvent. A method according to claim 23 or 24 wherein the composition is compressed to render it cohesive during extrusion, rolling or moulding to produce the article. A composition according to any of claims 1 to 13 which has been cured. An article according to claim 14 which has been cured. A composition according to claim 1 or an article according to claim 14 carried on a dimensionally heat-recoverable article so as to lie, in use, between the recoverable article and an object about which the article is to be recovered in use.

Inventors: ACOTT, KEVIN PETER, GENTILCORE, GIOVANNI, HADJIANDREOU, PAVLOS
Applicants: RAYCHEM LIMITED
Application year: 1986
Main classification: C08J3
IPC codes: C08L67, C08J5, C08L23, B29C61, C08L33, C08L63, C08L77, C08J3, C08G59, C08L101, C08G63, C09J7, C09J5, B29C43, F27D1, C08L7, C08L21