Coating processes and curable compositions

Abstract: Curable partially plasticised (optionally particulate) compositions comprising partially crystalline polymers which can be made by for example a non-attritive method comprising heating a mixture of the polymer in a preferably moderate curable solvent for the polymer to a temperature above the melting point of the polymer when in the solvent and then cooling whereupon the polymer re-crystallises from the mixture. Also the use of such compositions in coating and in coating formulation.

Description: This invention relates to coating processes in which a curable composition containing crystallisable polymer is provided on a surface and cured. It also relates to curable compositions containing crystallisable thermoplastic polymers (including copolymers and polymers optionally modified by the presence of additives for example other crystallisable or non-crystallisable polymers or grafted moieties such as those containing carboxylic moieties), to methods for making such compositions and to the use of the compositions in coating processes and formulations. Crystallisable polymers are well known to include polyesters such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT) which usually have a density of about 1.4 and 1.3 g/cm³ respectively, polycarbonates, polyamides (often called nylons) and polyolefins, especially low, medium and high density polyethylenes and isotactic polypropylenes. A fuller description of the various types of polyester, polycarbonate or nylon is given in the third edition of Kirk-Othmer's "Encyclopaedia of Chemical Technology" published by John Wiley & Sons of New York in 1982 see Volume 18 pages 549 to 574 pages 479 to 494 or pages 406 to 425 for polyesters, polycarbonates and nylons respectively or Volume 16 pages 402 to 441 or pages 453 to 467 for polyethylenes or polypropylenes respectively. These pages are herein incorporated by reference. Examples of useful polyester copolymers include polyethylene terephthalate, copolymers comprising both terephthalic and isophthalic acid and elastomeric polyesters having segments of a low glass transition temperature. Polyamides also include recently available nylon 4, 6 and so called partially crystalline aromatic nylons. Aromatic nylons are polyamides comprising condensates of aromatic diamines such as 1,3-di(aminomethyl) benzene. Examples of useful copolymers of ethylene include copolymers of ethylene with up to 30 wt% of other alpha-olefins or olefinically unsaturated carboxylic acids or esters such as vinyl acetate or lower (for example C₁ to C₄) alkyl acrylates or methacrylates, and especially polyethylenes containing small amounts (for example a trace to up to 1 wt%) of copolymerised or grafted carboxylic acid moiety. Useful propylene copolymers include copolymers comprising up to 15 wt% of ethylene. Polyolefins may be usefully blended with each other or up to 30 wt% of ethylene/propylene (optionally diene modified) rubbers or other rubbers. Other useful crystallisable polymers include polyvinyl fluorides, polyvinylidene fluorides, polyurethanes and polyhydroxy butyrates. Some of the above crystallisable polymers can aquire a crystalline form merely by solidifying from the molten state whilst some (notably the polycarbonates and some polyesters) are amorphous as supplied but may be easily converted to partially crystalline form by exposure to solvents. It is for this reason that the polymers are herein described generically as "crystallisable" rather than "crystalline". In general crystallisable thermoplastic polymers have many properties such as toughness, hardness, abrasion resistance and flexibility which make them useful as coating materials whereas conventional cured coatings can have advantages such as corrosion and solvent resistance and good adhesion to substrates. However, so far attempts to incorporate crystallisable thermoplastic polymers into cured coatings have led to highly inhomogenous structures which because of their inhomogeneity have shown only minimal improvements in the overall properties of the cured coating. A practical difficulty concerning the use of crystallisable thermoplastic polymers arises from the fact that many processes for their commercial manufacture produce pellets which have a maximum dimension of not less than 2mm which means that the pellets need to be converted into distinct fine particles, that is to say particles having a maximum dimension of below 500µm. Hitherto this has been done by attritive methods such as grinding or milling. However attritive methods produce particles of an unpredictable shape which are inconvenient to use in coating processes. It is therefore an object of this invention to provide a non-attritive process for making particles of crystallisable thermoplastic polymer containing entrapped curable solvent system and especially particles having a maximum dimension of below 500um. According to the present invention there is provided a non-attritive method for making particles from crystallisable polyester, polycarbonate polyamide or polyolefin polymer which comprises: (a) heating a mixture comprising (i) a curable solvent system which depresses the crystalline melt temperature (Tm) of the polymer by not more than 80°C when the polymer constitutes 20% by weight of a mixture of dry solvent system and dry polymer, and (ii) at least 5 wt% of the polymer (the percentage being based on the combined weights of the cureable solvent system and the polymer) to a temperature between 10 and 80°C above the crystalline melting point (Tm) of the polymer when in the mixture (b) cooling the heated mixture to a temperature below Tm so as to cause solid/liquid phase separation whereby particles of crystallisable polymer are obtained which comprise amorphous zones containing entrapped curable solvent system. This invention also provides a curable composition comprising the particles produceable by the above process. A further object of this invention is to provide a coating process which allows the production of cured coatings containing a more uniform distribution of crystallisable thermoplastic polymer with consequent improvement of properties. The coating process comprises of applying the composition to a surface and heating the composition so that the solvent system cures to form a cured coating. It is preferred that the polymer be in the form of dispersed particles having a number average maximum diameter of less than 500µm (and preferably less than 30µm, 1µm being 10⁻⁶µm). Such particles not only facilitate dissolution but also offer an alternative to complete dissolution of the polymer by the use of conditions (primarily less high temperatures and/or higher concentrations) which cause the particles to soften rather than dissolve. By "soften" it is meant that the particles become less hard than they would be in the absence of solvent system at ambient temperature and preferably soft enough to enable contiguous particles to coalesce. Softening is probably assisted by plasticisation by the entrapped solvent system. The use of softened particles as an alternative to dissolution reduces the risk of premature curing of heat-curable solvent systems and/or of thermal degradation of the polymer. Curable compositions provided on a surface may be heated to form a solution of the polymer and the curable solvent. Alternatively, if the composition comprises particles, it may be heated to soften particles of the polymer. Heating increased the ability of the polymer to flow which in turn gives coatings having better mechanical properties and improved appearance, particularly better gloss and/or smoothness. Curing may be initiated whilst the polymer is still in a solution or softened state or alternatively the composition may be cooled towards (preferably to) ambient temperature before any substantial curing is positively initiated. Curing will then occur in the presence of solid polymer and will have the effect of linking together adjacent amorphous zones of crystallisable polymer which generally improves homogeneity. Curing also similarly links together amorphous zones of polymer particles which have not been heated after they have been provided on the surface to be coated. Preferably the particles contain 10 to 50% by weight of the polymer. Particles obtained from the non-attritive method may sometimes be agglomerated. Where distinct particles are required, the agglomerate will need to be subjected to a shearing operation. A shearing operation differs from an attritive operation in that shearing merely separates particles which have already been formed whereas attrition forms particles by fracturing polymer. Often shearing can be achieved by forces no greater than those exertable between finger and thumb. A "dry solvent contains less than 0.005 wt% of water for polyesters and less than 0.05 wt% for polyamides and a "dry" polymer is a polymer which has been heated in a vacuum oven for 16h at 90°C. Crystalline melting point is determined by differential scanning calorimetry performed in turn on the polymer alone and on the above mixture. The mixture must be contained in a sealed capsule to prevent loss of the moderate solvent during heating. More particularly, 10mg of polymer alone and 10mg of mixture are each in turn subjected to cycles of heating and cooling performed under nitrogen in the calorimeter. Each heating/cooling cycle comprises heating the sample under test, (which may be polymer alone or a mixture) at a rate of 20°C/min to cause crystalline melting which occurs at a temperature T m , subsequently holding the sample for two minutes at a holding temperature T h which is above T m , then cooling at a rate of 20°C/min to cause recrystallisation which occurs at a temperature T c and finally continuing cooling to a temperature of at least 10°C below T c . T m and T c are detected respectively as an endothermic trough and an exothermic peak in the graph of heat absorbed or evolved versus temperature. Each cycle is repeated to discover whether a consistent value of T c can be obtained. If consistency is not obtained, another pair of heating/cooling cycles are performed using a slightly higher T h . Further pairs of cycles with gradually increasing T h are performed until consistent values for T c are achieved whereupon the pair of cycles which gave consistent values is repeated and the value for T m obtained is defined to be the crystalline melting point of the sample under test. Examples of suitable curable solvent systems (especially those suitable for use with polyesters) include epoxy ethers based on various glycidyl derivatives and in particular derivatives of bisphenol A available from Shell Chemicals Limited as "Epikotes". Such epoxy ethers have the general formula where n is preferably from 0 to 9 and may be non-integral, eg, 1.2. Other useful epoxy compounds include glycidyl derivatives of the following namely, bis(4-hydroxy phenyl) methane, terephthalic acid, isocyanurate, sorbitol and Novolac resins which are polyphenol formaldehyde. The curable epoxy compounds need to be used in combination with curing agents which bring about curing when heated. Suitable curing agents include dodecanoic acid, phthalic anhydride, diaminodiphenylsulphone, dicyandiamide, triethylene tetramine, 1,6 hexamethylene diamine and melamine or urea formaldehydes. The curable Novalac resins are particularly suitable for use with both polyesters and polyamides and need to be used in combination with curing agents such as hexamine and hexamethoxymethylmelamine. Polyamides may also be used with curable solvent systems such as glycerol or triethylene glycol which cure by reacting with curing agents such as formaldehyde resins. Unsaturated compounds such as long chain fatty acids (especially linseed oil) and dienes such as dicyclopentadiene may be used as curable solvent systems particularly with polyolefins. These cure by autoxidation preferably in the presence of catalysts such as cobalt octoate or naphthenate and accordingly no added curing agent is needed since atmospheric oxygen will permeate the composition. The curable solvent system may consist solely of a curable compound which is itself a solvent for the polymer. Alternatively it may comprise a curable compound present as a single phase mixture with a non-curable solvent in which case the curable compound need not necessarily be a solvent for the crystallisable polymer or it may be a poor solvent in which case the non-curable solvent may be regarded as an auxiliary solvent. For example the single phase mixture may be a solution of curable compound in non-curable solvent or a mixture of two miscible liquids one of which is the curable compound and one of which is non-curable solvent. Any non-curable solvent either should be tolerable in the cured coating (perhaps serving as a plasticiser) or should be sufficiently volatile to be removable from the coating composition after the composition has been provided on the surface being coated. It is essential when performing the non-attritive method for making particles of partially crystalline polymers that the mixture be heated to above the melting point (T m ) of the polymer when in the mixture for otherwise there will be obtained particles of unpredictable shape comprising some undissolved polymer. Preferably the mixture should be heated at least to its clearing temperature "T cl ". The "clearing temperature" (T cl ) of any chosen mixture comprising moderate solvent system and polymer is the temperature at which the appearance of the mixture becomes clear to the unaided eye. T cl is determined by heating 2g of a chosen mixture until the polymer dissolves and the initially turbid solution obtained turns clear for a first time, then cooling the mixture to room temperature an finally re-heating the mixture until it turns clear for the second time. The temperature at which it turns clear for the second time is defined to be the clearing temperature (T cl ) for that mixture. Heating to T cl and above (preferably to 10 to 30°C above T cl ) leads to the formation of more uniform particle sizes. Uniformity of particle size is also enhanced by heating the mixture to a holding temperature T h which is above T m for the polymer when in the mixture (and preferably 10 to 40°C or even 60°C above) and holding the mixture at temperature T h for from 1 to 60 minutes although holding for 1 to 5 minutes is usually sufficient. It has been found that some large pellets of polymer available commercially can be inconveniently slow to dissolve. Where time saving is important, this problem can be alleviated by using a pre-heating and pre-cooling cycle as follows. The mixture is first pre-heated to a temperature well above (for example 60°C above) the melting point of the pure polymer. Such pre-heating causes a rapid dissolution of the polymer. The mixture is then pre-cooled to at least a temperature at which the polymer re-solidifies from the mixture. For example the polymer is cooled to at least 30 to 80°C below the melting point of the pure polymer. Such re-solidification produces polymer in a form which dissolves quickly (usually within 2 minutes) on heating to T m or above so producing a mixture consisting of a solution of polymer in solvent system which appears clear to the unaided eye. Also in performing the non-attritive method, it is essential to employ conditions which cause solid/liquid phase separation to occur from the mixture of dissolved polymer and solvent system for otherwise a formless mass may be obtained. To achieve solid/liquid phase separation, it is necessary to use a cureable solvent system as defined above, to use a mixture containing at least 5 wt% of polymer and to avoid shock cooling which usually means cooling at a rate of no faster than 300°C/min and preferably no faster than 100°C/min. The size of the polymer particles can be adjusted by varying the cooling rates employed or by incorporating nucleating agents into the polymer. The nucleating agent chosen should not react with the solvent system. In general higher cooling rates and also nucleating agents favour smaller particles sizes. Typical nucleating agents for polyesters include talc, sodium benzoate or the ionomeric copolymers of ethylene with minor amounts of carboxylate comonomers, for example those known as "Surlyn" A available from EI Dupont de Nemours Inc. Typical nucleating agents for nylons include talc, fluorspar and those disclosed in British patent specification GB 1 465 046 the contents of which are herein incorporated by reference. Typical nucleating agents for polyolefins include talc, sodium benzoate or dibenzylidene sorbitol and its alkyl analogues. The crystallisable polymer may be mixed with the solvent system simply by adding the polymer to the system and then heating and stirring. However a convenient way to mix polymer and solvent system at elevated temperatures comprises feeding both polymer and solvent system to a closed shearing mixer and heating within the mixer to dissolve the polymer. The solution can then be delivered directly to the surface to be coated. The solution may be cooled in or out of the mixer to cause particles to form which can be subsequently applied to the surface. The preferred mixer is an extruder and the extrudate solution may be applied directly to a surface or cooled to form particles. Mixing in an extruder is particularly useful in performing the heating steps in the non-attritive processes for making particles containing entrapped solvent. Extruders offer good control of temperature and so heating can be performed with minimal risk of premature curing of heat-curable solvent systems. A shearing mixer can also be used to promote the grafting of suitably reactive moieties onto the crystallisable polymer and in particular suitably reactive curable solvent systems may be so grafted with consequent improvements in compatibility between crystallisable polymer and cured coating. Returning to the coating process according to this invention, the curable composition is eventually subjected to conditions under which the solvent system cures to form the cured coating. Conventional curing techniques are employed. For example a heat-curable composition may be transferred to an oven heated to the preferred temperatures for curing. An autoxidisable composition may be allowed to stand in air. Other compositions may be exposed to curing vapours or liquids. Usually the weight ratio of crystallisable polymer to curable compound in the composition provided on the surface is from 1:0.1 to 19. Towards the higher ratios of polymer to compound (often at ratios of above 1:4 especially 1:3 and above, the polymer may not always dissolve completely during coating but instead it becomes a mass (usually a paste) of polymer particles softened by solvent system which later cohere for example if the composition is heated to a temperature at which softening or dissolution begins to take place. Most preferably such heating will be to a temperature which is above the temperature (T m ) at which crystalline melting occurs in that composition. The composition will generally be held at these elevated temperatures for from 2 to 45 minutes. It has been found that when the weight ratio of crystallisable polymer to curable compound is from 1:0.25 to 4, the cured coating usually comprises a network of cohering portions of crystallisable polymer interpenetrated by a network of cohering portions of cured compound. Where the amount of cured polymer is greater than 1:4, the tendency is for the cured coating to comprise a homogenous dispersion of particles of crystllisable polymer in and cohering to cured compound and vice versa if the amount of cured polymer is less than 1:0.25. Interpenetrating networks confer optimum combinations of the properties of crystallisable polymers and cured resins. In particular they increase the ability of surfaces coated with cured resins to undergo shaping operations. The compositions may be applied to surfaces by a wide variety of conventional techniques such as doctor bar coating, roller coating, spray coating, extrusion coating, brushing and dip coating. Particles may be applied as a dispersion in curable solvent system. Where the particles comprise amorphous zones containing entrapped curable solvent system, they may contain enough entrapped solvent to render the presence of further solvent unnecessary in which case they may be applied as a free flowing powder or as a paste or as a paste or dispersion comprising a non-solvent carrier liquid. The compositions may be applied to opposed surfaces in which case they serve as adhesives. The surfaces coated according to this invention may be metallic for example aluminium, stainless steel or non-metallic, for example glass, wood, paper or textile. In particular the particles may be used to coat sheets (especially sheets which are to be used in shaping processes), and shaped articles such as cans. The compositions may be used to impregnate continuous rovings of a wide variety of fibres including glass and carbon fibres by application to the surface of the fibres. The impregnated fibres, may be shaped druing curing to produce a composite which on cooling comprises fibre consolidated in cured solvent. The compositions can be used as adhesives to provide laminates and especially laminates for use in shaping processes. It also has been found that the particles of crystallisable polymer comprising amorphous zones containing curable solvent disperse well in coating compositions (for example paints and varnishes) based on water or organic solvents. Accordingly this invention provides a coating composition (which may be based on an organic solvent or water) comprising a binder of a type used in coating compositions and particles of polymer wherein the particles of polymer comprise zones of amorphous polymer which contain the entrapped curable moderate solvent. The binder may be a curable resin which is co-curable with the solvent system. Typical binders for coating compositions are described in the third edition of the book "Introduction to Paint Chemistry and Principles of Paint Technology" by G P A Turner and published by Chapman and Hall of London in 1988, the contents of which are herein incorporated by reference. The coating composition may also comprise pigments and extenders and other conventional ingredients described in the above book. Crystallisable polymer (preferably in particulate form) comprising amorphous zones containing entrapped curable solvent system may also be used in a wide variety of thermoforming operations such as extrusion, compression or injection moulding provided that conditions are chosen so as to avoid premature curing of the entrapped solvent. The invention is further illustrated by the following Examples in which the following tests are employed: The Rubbing Test: In the Rubbing Test, each coating was rubbed back and forth with a lambswool cloth soaked in methyl ethyl ketone. After a number of back and forth rubs, the coated surface became visible through the coating when viewed through an optical microscope at ten fold magnification. The number of back and forth rubs needed for this to happen is recorded. The Chisel Test: In the Chisel Test, the apex of an isoceles triangular blade was stood on the coating with the blade touching the coating and subtending an angle of 45°C thereto. The base of the triangle was 6.1mm long and the distance from the base to the apex was also 6.1mm. The chisel was loaded with a weight and then drawn once across the coating in a direction parallel to the 300mm edge of the sheet. The loading weight was increased until the apex of the blade caused the aluminium surface to become visible through the coating when viewed through an optical microscope at ten fold magnification. The loading needed for this to happen is recorded. The "T" Bend Test: The "T" Bend Test is performed according to the procedure of ASTM Test D 4145-83 the contents of which are herein incorporated by reference. The Corrosion Test: The Acetic Acid-salt Spray Corrosion Test is performed according to the procedure of ASTM Test B 287-74 the contents of which are herein incorporated by reference. The Reverse Impact Test: The Reverse Impact Test is performed according to the procedure of ASTM Test D 4145-83 ("Resistance of Organic Coatings to the Effects of Rapid Deformation") the contents of which are herein incorporated by reference. Also in the Examples coatings are applied to flat smooth sheets using a Meyer doctoring bar which comprises a cylindrical metal bar of diameter 9mm around which is helically wound a single layer of a wire of diameter 0.8mm with its adjacent helices contiguous throughout their length. In this way the bar is provided with a series of circumferential parallel grooves each 400µm deep. To apply a coating, the bar sheet and the curvarture of the bar combine to define a nip into which the composition to be applied is delivered. The bar is then drawn across the sheet leaving a trail of parallel ridges of the composition initially 400µm high. EXAMPLES 1 TO 20 Making and Use of Curable Compositions containing Polyester Particles: 10g samples of various polyesters specified in Table 1 were each mixed with a curable epoxy moderate solvent system which was "Epikote" 880 (which is bisphenol A diglycidyl ether) in amounts also specified in Table 1. Each mixture was heated to a temperature which was above the crystalline melting point T m of the polyester when in the mixture and also 50°C above the clearing temperature T cl of the mixture so as to dissolve the polyester whereupon the mixture existed as a clear solution. T cl for each mixture is given in Table 1. The mixture was held at this temperature for 2 minutes and then cooled to a temperature 50°C below T cl . A solid/liquid phase separation and re-crystallisation occurred. The heating and cooling cycle was repeated three times except that the third cooling was allowed to proceed to room temperature thereby producing fine, distinct and approximately spherical partially plasticised particles comprising crystalline zones Eg Polymer Amount Polymer in Mixture, wt % T cl °C Typical Particle Dimension µm 1 PET 10 195-200 10 2 PET 20 195-200 10 3 PET 30 195-200 10 4 PET 40 200 10 5 PBT 10 170 5-10 6 PBT 20 170 5-10 7 PBT 30 170 5-10 8 PBT 40 170 5-10 9 PET/I 10 135-140 5-10 10 PET/I 20 135-140 5-10 11 PET/I 30 135-140 5-10 12 PET/I 40 135-140 5-10 13 Hytrel 4056 10 95-100 2-5 14 Hytrel 4056 20 95-100 2-5 15 Hytrel 4056 30 95-100 2-5 16 Hytrel 4056 40 95-100 2-5 17 Hytrel 7246 10 195-200 2-5 18 Hytrel 7246 20 195-200 2-5 19 Hytrel 7246 30 195-200 2-5 20 Hytrel 7246 40 195-200 2-5 and amorphous zones containing imbibed curable epoxy solvent system. A typical dimension of the particles as determined by optical microscopy is shown in Table 1. The particles comprised about 50 wt % of entrapped reactive solvent and were obtained as a paste consisting of particles and moderate solvent. 10g of each of the above pastes were in turn introduced to curing agent by thoroughly mixing with (as shown in Table 2) the curing agent which was diaminodiphenylsulphone. The weight of curing agent used was equal to half the weight of "Epilote" 880 used. Each paste in turn was then applied to a flat smooth aluminium sheet 300mm long by 100mm wide using a Meyer doctoring plate. Next the sheets were heated in an oven for 45 minutes and at a temperature in the case of Examples 5 to 16 of 200°C and in the case of Examples 1 to 4 and 17 to 20 of 230°C. The sheets were then allowed to cool to room temperature whereupon they were found to carry a smooth glossy tenacious coating of cured synthetic material. These coatings were then subjected to the rubbing and chisel tests and the results are shown in Table 2. The pastes obtained could be converted to dry free flowing particles by rinsing with acetone to remove moderate solvent system external of the particles. In Table 1 the polymers used were as follows: The PET had an intrinsic viscosity of 0.64 to 0.66 cm³/g in orthochlorophenol at 25°C. The PBT was "Celanex" 2002-2 from the Celanese Company. PET/I is a PET copolymer containing 18 wt% isophthalate. "Hytrels" are available from EI DuPont de Nemours. Example Wt of Curing Agent g No. of rubs to expose metal Chisel load to expose metal g 1 3.0 >100 400 2 2.6 >100 500 3 2.3 >100 300 4 2.0 >100 300 5 3.0 >100 200 6 2.6 >100 100 7 2.3 >100 200 8 2.0 >100 200 9 3.0 >100 300 10 2.6 >100 200 11 2.3 >100 200 12 2.0 >100 300 13 3.0 >100 200 14 2.6 >100 100 15 2.3 52 >100 16 2.0 6 200 17 3.0 >100 400 18 2.6 >100 300 19 2.3 >100 300 20 2.0 >100 300 EXAMPLE 21 Making and Use of Curable Composition containing Polyamide Particles: 3g of a nylon 6 polyamide available as "Maranyl" B3 from Imperial Chemical Industries PLC was mixed with 5g of a curable moderate solvent system which was a "Novalac" phenolic resin having a melting point 108°C and available from Schenectady Midland Chemicals Ltd and with 5g of benzyl alcohol as auxiliary solvent. The mixture was heated to a temperature of 200°C which was above the crystalline melting point T m of the nylon when in the mixture and also above its clearing temperature T cl so as to dissolve the nylon whereupon the mixture existed as a solution. The mixture was held at 200°C for 2 minutes and then cooled to 120°C. The mixture was heated to 200°C for a second time and then allowed to cool to 50°C. Finally the mixture was heated to 200°C for a third time and then allowed to cool to room temperature and stood in iced water for 5 minutes whereupon a solid/liquid phase separation and re-crystallisation occurred producing fine, distinct and approximately spherical partially plasticised particles comprising crystalline zones and amorphous zones containing entrapped solvent system. A typical dimension of the particles as determined by optical microscope was from 10 to 15µm. The particles comprised about 50 wt% of entrapped solvent system and were obtained as a paste consisting of particles and moderate solvent system. The pastes obtained could be converted to dry free flowing particles by rinsing with acetone to remove solvent system external of the particles. Rinsing amounted to placing the paste on filter paper in a funnel and pouring acetone through for 30 seconds at room temperature. The pastes were also suitable for mixing with hexamethoxymethyl melamine to produce a curable composition which could be cured in a hot oven to give smooth glossy tenacious coherent coatings on aluminium. EXAMPLES 22 AND 23 Making and Use of Curable Compositions containing Polyethylene Particles: High density polyethylene available from BP Chemicals Ltd as "Rigidex" HM 5590 EA was mixed with a curable moderate solvent system which was linseed oil in amounts as specified in Table 3. 10g of each mixture was subjected to a double pre-heating/pre-cooling cycle and then heated to a temperature above the crystalline melting point of the polyethylene when in the linseed oil and also above the clearing temperature for the mixture. The mixture was finally allowed to cool to room temperature. The temperatures reached in these heating cycles are given in Table 4. Heating caused the polyethylene to dissolve whereupon the mixture existed as a solution. At least the last cooling caused a solid/liquid phase separation and re-crystallisation which produced fine, distinct and partially plasticised particles comprising crystalline zones Eg Amount PE in Mixture, wt % Typical Particle Dimension µm 22 10 2-6 23 30 2-6 HEATING CYCLES Amount PE wt% 1st Pre-Heat to °C Cool to °C 2nd Pre-heat to °C Cool to °C Heat to °C 10 220 52 210 60 210 30 220 60 220 70 210 and amorphous zones containing entrapped linseed oil. The particles were either spherical or oblate having a typical maximum dimension as determined by optical microscope as shown in Table 3. The particles comprised about 50 wt % of imbibed linseed oil and were obtained as a paste consisting of particles and linseed oil. The pastes obtained can be mixed with a conventional amount of cobalt naphthenate to produce a curable composition suitable for curing in a hot oven to give smooth glossy tenacious coherent coatings on aluminium. EXAMPLES 24 TO 27 Making and Use of Curable Compositions containing Polypropylene Particles: 10g samples of a polypropylene (PP) were each mixed with a curable moderate solvent in amounts as specified in Table 5. The polypropylene was available from Imperial Chemical Industries PLC as "Propathene" GWE 26 and had a melt flow index of 3 as determined by BS 2782720A using a 2.16kg load at 230°C. Each mixture was subjected to a double pre-heating/pre-cooling cycle and then heated to a temperature above the crystalline melting point of the polymer when in the solvent system and also above the clearing temperature for the mixture. The mixture was finally allowed to cool to room temperature. The temperatures reached in these heating cycles are given in Table 6. Heating caused the polymer to dissolve whereupon the mixture existed as a solution. At least the last cooling caused a solid/liquid phase separation and re-crystallisation which produced fine, distinct and approximately spherical partially plasticised particles comprising crystalline zones and amorphous zones containing Eg Solvent Amount Polymer in Mixture, wt % Typical particle size µm 24 Linseed oil 10 6-10 25 Linseed oil 30 6-10 26 Dicyclopentadiene 10 12 27 Dicyclopentadiene 30 12 to 15 entrapped curable solvent system. The particles comprised about 50 wt % of entrapped solvent system and were obtained as a paste consisting of particles and solvent system. Typical particle sizes obtained as determined by optical microscopy are shown in Table 5. The particles were mixed with cobalt naphthenate to produce a curable composition which was suitable for curing in an oven to produce smooth glossy tenacious coherent coatings on aluminium. EXAMPLE 28 Making and Use of Curable Compositions containing Polyester and Curable Oligomeric Solvent System: 2.2g of PBT (available as "Valox" 310) was mixed with a curable oligomeric moderate solvent system which was 22g of an oligomeric curable compound which was a condensate of a mixture of terephthalic acid, isophthalic acid, adipic acid trimethylylol propane and neopentaglycol in the molar ratios of 0.51 : 0.09 : 0.4 : 0.12 : 0.98 having an weight average molecular weight of about 1900. The mixture was heated to 220°C, allowed to cool to 25°C and then re-heated to 240°C and allowed to cool to 100°C whereupon re-crystallisation occurred. At 100°C, the mixture was diluted by the addition of an equal weight of an auxiliary solvent which was a 1:1 mixture of "Solvesso" 150 and methyl propoxol acetate. ("Solvesso" 150 is a mixture of aromatic C₉ and C₁₀ hydrocarbons having a boiling point range of 190 to 210°C at 1 bar). The mixture then cooled to room temperature. A curing agent system consisting of 1.85g of hexamethoxymethyl melamine and 0.14g of a sulphonic acid available as "Nacure" XP 253 was added to 23.35g of the above mixture and the whole stirred. The stirred mixture was then applied to a smooth aluminium sheet using the Meyer doctoring bar and heated in an oven for 60 secs at a peak temperature of 232°C. A smooth glossy tenacious coherent cured coating was obtained on the aluminium. EXAMPLE 29 Making and Use of curable Compositions containing Polyester Elastomer and Curable non-Solvent: A mixture of 72.3g polyester curable elastomer available as "Hytrel" 4056 and 108.5g of the oligomer used in Example 28 was heated to 220°C, held for 10 minutes at 220°C, then allowed to cool to 200°C and then 69.7g of auxiliary solvent which was "Solvesso" 150 was added. The mixture was next allowed to cool to 150°C when 50.8g of a second auxiliary solvent which was methyl propoxol acetate was added. The mixture was allowed to cool to 50°C and then re-heated to 90°C and 100g of a 1:1 mixture of "Solvesso" 150 and methyl propoxol acetate was added. The mixture was finally allowed to cool to room temperature during which cooling re-crystallisation occurred. 50g of the re-crystallised mixture was stirred with 2.126g of hexamethoxymethyl melamine curing agent, 0.28g of the sulphonic acid curing agent used in Example 28 and a further 25g of "Solvesso" 150 to produce a curable composition. The composition was applied to a smooth flat aluminium sheet as in Example 28 and heated in an oven for 60 seconds to a peak temperature of 232°C. A smooth glossy tenacious coherent cured coating was obtained on the aluminium sheet. EXAMPLES 30 TO 35 AND COMPARATIVE EXAMPLE A Effect of Varying the Polyester content of Curable Compositions: PBT similiar to that used in Examples 5 to 8 but supplied by ATO Chemic as "Orgator" TMNO was mixed with a moderate curable solvent system which was "Epikote" 880 bisphenol A diglycidyl ether to give mixtures containing amounts of PBT as specified in Table 7. Each mixture was heated to a temperature which was above the crystalline melting point T m of the polyester when in the mixture and also 50°C above the clearing temperature T cl of the mixture so as to dissolve the polyester whereupon the mixture existed as a clear solution. T cl for each mixture is given in Table 7. The mixture was held at this temperature for 20 to 40 minutes and then cooled to a temperature 50°C below T cl . A solid/liquid phase separation phase separation occurred. The heating and cooling cycle was repeated and then cooling was allowed to proceed to room temperature. Examples 30 to 33 produced particles whereas Examples 34 and 35 produced a hard mass which was ground into particles using the mill described in Example 36. The cooled mixtures were mixed with curing agent which was diaminodiphenylsulphone. The amount of curing agent added was equal to half the weight of the "Epikote" used. The mixtures were then re-heated to a temperature above their T cl and applied to smooth aluminium sheet 300mm by 100mm using the Meyer doctoring bar. The sheets were heated in an oven at 230°C for 30 minutes to cure the compositon and then allowed to cool to ambient temperature (20°C). Smooth glossy tenacious coherent coatings were obtained. The above procedure was repeated except that the coated sheets were forced cooled after curing by quenching in cold water at a temperature of 20°C. Again smooth glossy tenacious coherent coatings were obtained. Both quenched and unquenched coated sheets were subjected to various tests and the results are shown in Table 3 where the suffix "q" denotes the results obtained from the quenched samples. The chisel test loading shown is the minimum loading needed to cause the aluminium sheet to become visible through the coating. The T bend result shows the minimum number of bends at which no fracture of the coating is visible to the unaided eye. For example Example Wt% PBT in mixture T cl Film Thickness µm Chisel Test Loading g T Bend A 0 - 22 200 2 Aq 0 - 16 200 2 30 10 210 20 200 1 30 10 210 23 100 1 31 20 208 20 200 1 31 20 208 20 200 1 32 30 218 23 200 1 32 30 218 19 200 0 33 40 218 27 200 0 33 40 218 20 300 0 34 50 220 20 300 0 34 50 220 17 300 0 35 75 225 16 300 0 35 75 225 17 200 0 a result of 0 (according to ASTM reporting practice) means that the coated sheet can be bent back onto itself with no fracture occuring at the bend. Bending back onto itself is the most severe test because the curvature at the bend is greatest. Results of 1,2 or more mean that the coated has to be bent back onto 1,2 or more thicknesses of coated sheet in order to escape fracture at the bend whereupon the curvature is accordingly reduced by 50%, 33.33% or more respectively and so the test is less severe. The T-bend results show that compositons containing more that 20 wt% PBT are capable of being back onto themselves which indicates that they are highly suitable for use in forming processes. When such coatings were etched with concentrated (95%) sulphuric acid for 1 to 2 minutes to remove the epoxy material and photographed at a mignification of 5000 using an electron microscope, it could be seen that the particles of PBT where contiguous and coalesced so forming a largely continuous network of PBT which had permeated the epoxy material. Provided that the amount of PBT did not exceed 75 wt%, the epoxy material likewise formed a network which permeated the PBT networks. A qualitative inspection of corrosion tests showed that coatings having interpenetrating networks showed superior corrosion resistance. The photographs also showed that as the amount of PBT reached 30 wt%, the coalescence between particles increased giving the PBT network a structure resembling a network of filaments. Increasing the amount of PBT further increases the degree of coalescence until at amounts of 75 wt and above, the coating comprises particles of epoxy material dispersed in crystalline polymer. The various coated sheets were subjected to the rubbing test and all were found to survive over 100 rubs. EXAMPLE 36 AND COMPARATIVE EXAMPLE B Use of Alternative Curable Solvent System: A mixture consisting of 20 wt% PBT and 80 wt% "Epikote" 1001 (curable solvent) was made. "Epikote" 1001 is a glycidyl derivative of bisphenol A having the general formula shown earlier where n gives a number average molecular weight of about 900. It has a melting point of from 64 to 76°C and so is solid at ambient temperatures and accordingly needs to be melted to permit mixing to occur. Mixing was in fact performed at 80°C. The molten mixture was heated to 260°C (which is 50°C above its T cl ), cooled t 160°C, re-heated to 260°C and then allowed to cool to ambient temperature (20°C) producing a solid mass of "Epikote" containing a uniform dispersion of particles which had coalesced to form a network. The solidified mass was ground into particles using a Retsch ultracentrifugal mill as supplied by F Kurt Retsch of Haan in West Germany and as described in their brochure No 99.785.0002 of September 1987. The solid ground particles were mixed with curing agent, curing accelerator and volatile carrier liquid to form a curable paste which was convenient to apply to a surface. The curing agent was "Cymel" 303 (hexamethoxymethyl melamine) supplied by CIBA-Geigy, the accelerator was a 1 wt% solution of a blocked dinonyl naphthalene disulphonic acid (Nacure X 49-110) in butoxy ethanol and the carrier liquid was methyl ethyl ketone. The paste contained 6.7g of the ground particles, 1.07g of "Cymel" 303, 0.54g of the accelerator solution and sufficient ketone to give the paste a non-volatile content of 20 wt%. The paste was applied to a pair of aluminium sheets 300mm by 100mm, the sheets were heated to 225°C for 10 minutes and then one was allowed to cool to ambient temperature (20°C) and the other was force-cooled by quenching in cold water at a temperature of 20°C. The cooled sheets were subjected to various tests and the results are shown in Table 8. For Comparative Example B, the above coating procedure was repeated but using "Epikote" 1001 which did not contain PBT. The results are also shown in Table 8. Example Wt% PBT in mixture T cl Film Thickness µm Chisel Test Loading g T Bend Reverse Impact kg.m B 20 208 16 200 1 0.46 36 20 208 18 200 1 0.92 Use of an extruder: Equal weights of PET (as used in Examples 1 to 4) and "Epikote" 1004 (curable solvent) were simultaneously fed to an extruder at a rate of 20g each per minute. "Epikote" 1004 is a glycidyl derivative of bisphenol A having the general formula shown earlier where n gives a number average molecular weight of about 1400. The extruder was a Werner Pfleiferer ZSK30 twin screw machine operated with a screw speed of 100 rpm. The barrel of the extruder was heated to 270°C so that the PET dissolved in the "Epikote". On extrusion from the extruder, the mixture was allowed to cool to ambient temperature (20°C) whereupon the "Epikote" solidified producing a solidified mass of "Epikote" containing uniform dispersion of particles of PET coalesced to form a network. This mass was ground into particles using the technique of Example 36. The ground particles were mixed with the curing agent, accelerator and carrier liquid used in Example 36 and converted to a paste containing 20 wt% of non-volatile material. The amount of curing agent used was equal to 10 wt% of the "Epikote" present and the amount of accelerator was equal to 0.5 wt% of the curing agent. The paste was applied to an aluminium sheet 300mm by 100mm, heated to 255°C for 10 minutes to cure the "Epikote" and then allowed to cool to ambient temperature (20°C). A smooth glossy coating was obtained. EXAMPLE 38 Application of Curable Composition by Spray Coating: 40g of PBT (as used in Examples 5 to 8) were added to 60g of a curable polyester solvent system which was "Arakote" 3109 which is supplied by CIBA-Geigy. The PBT and resin were heated to 240°C, maintained at that temperature for 30 minutes and then allowed to cool to room temperature whereupon a solid mass of "Arakote" was obtained containing a uniform dispersion of particles of PBT coalesced into a network. The solid mass was ground into particles as in Example 36 and the particles were mixed with a pre-ground blocked isocyanate curing agent which was supplied as B 1530 by Chemische Werke Huls. The amount of curing agent used was 16 wt% of the weight of the "Arakote". The curable mixture so obtained was then electrostatically sprayed onto aluminium panels 300mm by 100mm, heated to 230°C for 20 minutes in an oven and allowed to cool to ambient temperature (20°C). Good smooth coherent coatings were obtained. EXAMPLE 39 Composition Curable at Ambient Temperature: 11g of Nylon 66 was mixed with 25.6g of glycerol, heated to 225°C for 10 minutes and then allowed to cool to ambient temperature (20°C) whereupon a hard paste was obtained. The paste was ground in a pestel and mortar with a) 3g of a copolymer consisting of 70wt% hydroxyethyl acrylate copolymerised with 30wt% of ethyl acrylate b) lg of "Cymel" 1172 which has the structure shown below and c) lg of para toluene sulphonic acid. The resultant mixed paste was applied to a glass and an aluminium panel each 300mm by 100mm using the Meyer bar. The panels were then allowed to stand at room temperature for 10 hours whereupon both acquired smooth glossy tenacious coherent cured coatings. EXAMPLE 40 Making and Use of Curable Composition containing Polycarbonate: Amorphous bisphenol A polycarbonate available as "Sinvet" 251 from ENI Chem was mixed equal volumes of a curable solvent system which was "Epikote" 880 and of an auxiliary solvent which was dimethyl phthalate to produce a mixture consisting of 40wt% polycarbonate and 60wt% of the solvents. The mixture was heated to 150°C for two minutes and allowed to cool to ambient temperature (20°C). A portion of the cooled mixture was re-heated to 150°C and again allowed to cool to ambient temperature whereupon particles were obtained which comprised amorphous zones containing entrapped solvent and had a particle size of from 2 to 10µm. The particles could be mixed with diaminodiphenylsulphone curing agent and applied to aluminium sheets as in Examples 1 to 4 to produce glossy smooth tenacious coherent coatings.

Claims: A non-attritive method for making particles from crystallisable polyester, polycarbonate polyamide or polyolefin polymer which comprises; (a) heating a mixture comprising (i) a curable solvent system which depresses the crystalline melt temperature (Tm) of the polymer by not more than 80°C when the polymer constitutes 20% by weight of a mixture of dry solvent system and dry polymer, and (ii) at least 5 wt% of the polymer (the percentage being based on the combined weights of the cureable solvent system and the polymer) to a temperature between 10 and 80°C above the crystalline melting point (Tm) of the polymer when in the mixture (b) cooling the heated mixture to a temperature below Tm so as to cause solid/liquid phase separation whereby particles of crystallisable polymer are obtained which comprise amorphous zones containing entrapped curable solvent system. A method according to Claim 1 in which the polymer is mixed with the solvent system in a closed shearing mixer. A method according to Claim 2 in which the mixer is an extruder. A method according to any one of Claims 1 to 3 in which the weight ratio of crystallisable polymer to curable material comprised by the solvent system is from 1:0.1 to 1:19. A curable composition comprising particles of crystallisable polymer producible by the process of Claim 1. A curable composition according to Claim 1 in which the particles have a maximum diameter of less than 500µm. A coating process which comprises applying a curable composition according to Claim 5 to a surface and heating the curable composition so that the solvent system cures to form a cured coating. A process according to Claim 7 in which for a portion of the time during which the process is being performed and the curable composition is on the surface, the curable composition is at or reaches a temperature at which the polymer is dissolved or softened. A process according to Claim 7 or Claim 8 in which the curable composition is heated so that the solvent system cures whilst there is present in the composition solid polymer comprising zones of amorphous polymer containing curable solvent system. A process according to Claims 7 wherein the weight ratio of the crystallisable polymer to the curable material comprised by the solvent system in the curable composition is from 1:0.25 to 1:4 whereby the cured coating produced comprises interpenetrating networks of crystallisable polymer and cured solvent system. A cured coating producible by the process of Claim 7 A cured coating producible by the process of Claim 10 comprising interpenetrating networks of crystallisable polymer and cured solvent system.

Inventors: BOVEY DENIS MALCOLM HARVEY, NIELD ERIC, SUEMUL DALJIT KAUR, BOVEY, DENIS MALCOLM HARVEY, NIELD, ERIC, SUEMUL, DALJIT KAUR
Applicants: ICI PLC, IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1989
Main classification: C09D201
IPC codes: C08J3, B05D7, C08L101, C08J5, C09D201, C09D177, C09D167, C09D163, C09D123