HEAT-ACTIVATABLE POLYURETHANE COATINGS AND THEIR USE AS ADHESIVES

Abstract: A coated sheet has a heat-activatable adhesive coating of thermoplastic polyurethane having a melting point of 40 to 100 DEG C. The coating may be crystalline and have an average crystal size below 10 mu m. The coating, when molten, may remain tacky for a period of from 2 to 50 seconds. Preferred processes comprise heat activating the coating and subsequently cold or cool sealing, for instance to form a packet around a heat-sensitive material.

Description: 1 HEAT-ACTIVATAB E POLYURETHANE COATINGS AND THETR USE AS ADHESIVES This invention relates to sheets coated with heat-activatable coatings, their production, and their uses in forming sealed packages and in lamination applications. It is well known to provide heat-activatable adhesive coatings on films and other sheets. These heat-activatable coatings can either be for heat sealing or cold sealing. A heat-sealable, heat-activatable, coating is one which has to be heated to render it tacky and is pressed against the surface to which it is to adhere while it is still tacky. Often the heating and the pressing are conducted simultaneously using heated jaws. A cold-sealable, heat-activatable, coating is one which is activated by heating generally until it is tacky, it is then allowed to cool during which time its tackiness may decrease, and is then pressed without significant heating against a receiving surface to which it adheres. The jaws or other device for causing the pressing are usually at ambient temperature (e g., 25°C) but may be heated slightly, e.g., up to 30 or 35°C, or may be cooled (e. , to 0°C or below, for instance when packaging ice cream or other frozen or chilled products). Generally the receiving surface is another coating of the same material. Good sealing usually requires that the cold sealing is effected before the coating loses all its tacky feel Heat-activatable adhesive coatings formed mainly of thermoplastic polyurethane are described in EP-A-574803 and, especially, in CA-A-2174288. The normal way of applying the coating to the film or other sheet material is by coating a liquid coating composition containing the appropriate coating solids onto a sheet as the sheet travels continuously past a coating station, evaporating the liquid carrier, and thereby forming a tacky coating, cooling the tacky coating to render it non tacky, and then winding the coated sheet into a roll. It is important that the coating does not block or otherwise bond to the reverse face of the sheet in the roll since otherwise it is difficult, inefficient or impossible to unwind the roll. In practice therefore it is generally desirable that the coating should lose adhesive activity as fast and as fully as possible during the cooling step, so as to minimise the risk of blocking. When the coating is to be used for hot sealing, it is desirable that it should be capable of being heated easily and quickly to form a tacky coating, and that this should give a good seal when hot pressed to the receiving surface It is an unfortunate fact that coatings that give a good seal under these conditions tend to have tend to be rather slow at losing their sealing activity during the cooling step in initial manufacture, and may have some tendency to block in the roll. It is particularly difficult to obtain a good balance between blocking and sealing when the coating is to be used for cold sealing. Cold sealing is particularly important when the adhesive is to be applied to a packet containing heat sensitive material, such as chocolate or other low melting material, ice cream or other chilled or frozen material, or various other food stuffs. If heat sealing were to be used, the heat from the heated jaws or other means for heat sealing might damage the material in the package. Accordingly it is desirable to activate the coating by heating at one station in the production line, insert the heat sensitive material in the package at a subsequent station, and then cold seal around the packaged material at a third station (while the coating is still activated). If the coating is to give a good cold seal, it must remain active for a significant period after the activation station. Unfortunately this means it will remain active for a significant period at the cooling stage during initial manufacture Accordingly coatings which give a good cold seal have a tendency to give major blocking problems when the film is initially coated and wound-up during initial manufacture. If blocking is low and satisfactory, an inadequate seal tends to be obtained when the time between the heating station and cold sealing station is sufficiently long to avoid damage to the heat sensitive ingredients. There is therefore a need for an optimum compromise between blocking properties and sealing properties. It is described in CA-A-2, 174,288 that the described coatings can include hydrophobic auxiliaries and various commercial auxiliaries and additives. It is said that the packaged film can be used for heat sealing or that it can be used by heat-activating the film and then simultaneously or within 10 minutes cooling below the melting point and cold sealing it. Although it is also asserted that the film surfaces can, after cooling, be brought into contact with one another without adhesion and low blocking values were recorded in a laboratory test, it also warns that rolling up the film with a release paper may be necessary to avoid blocking if the cooling is inadequate during manufacture. Cold sealing properties and low blocking are relatively easy to achieve when the processing conditions and materials are relatively non-challenging, for instance when the coating equipment is very lengthy and/or when the film speed during initial manufacture is relatively slow (so that there is plenty of time for thorough deactivation before the film is wound up), or when the reverse face of the film is repellent to the coating. However we have found that the polyurethanes described in CA-A-2, 174,288 cannot satisfactorily be used on many substrates simultaneously to give low blocking during high speed commercial manufacture of a roll on the one hand and subsequent good heat sealing or cold sealing on the other. On many substrates, if good heat or cold sealing is achieved then blocking in the roll is a problem, but if blocking is not a problem then cold or hot sealing tends to be inferior The balance between blocking and sealing is influence by the nature of the reverse face of the coated film (i.e. the face which is in contact with the coating when the film is wound-up on the roll). Blocking tendency may be low with, for instance uncoated polypropylene but may be high when the surface has polar properties, for instance of the type that are generally required to make that face printable by convenient printing inks. For instance blocking may be a particular problem when the reverse face is coated with a hydrophilic acrylic coating. Although there have been other proposals for heat-activatable, cold-sealable coatings, none have proved satisfactory (especially for packaging heat-sensitive materials such as foodstuffs) and may introduce other problems such as solvent and/or the use of contamination, chlorinated polymers, blocking. It has been our object to provide coated sheets which are heat-activatable and which give an improved combination of low blocking tendency with good sealing properties and, especially, good cold sealing properties. In particular, it has been our object to provide such coated films using high speed, compact, coating and sealing equipment. In the invention we provide a coated sheet carrying a heat-activatable adhesive coating of coating solids which are mainly thermoplastic polyurethane, and have a melting point of 40 to 100°C. In a first aspect of the invention, the coating on the sheet is crystalline and has an average crystal size of below 10 μm The actual crystal size is influenced to some extent by the thickness of the coating and the conditions by which the cooled crystalline coating is achieved. Instead of or in addition to defining the coating in terms of their crystal properties, the coating can be defined in terms of the time the coating, after being molten, remains tacky when cooled at 25°C. In this second aspect of the invention, we provide a coated sheet wherein the coating remains tacky when pressed on itself, for at least 2 seconds but not more than 50 seconds when cooled at 25°C from the molten state. Thus a molten coating of the desired thickness on the desired substrate is formed at or just above the melting point of the compositions, and the time for the coating to become non-tacky when exposed to an ambient atmosphere at 25°C is measured. In particular, the coating is usually molten in an oven at 100°C and is then exposed to an ambient temperature of 25°C. The methods for determining crystal size and tack are described in more detail below. The preferred coating on the sheet is a coating which remains tacky for at least 5 seconds but not more than 50 seconds upon cooling at 25°C from the molten state. In general, this period of tackiness seems to be associated with the preferred crystal sizes, which are preferably in the range 0.5 to 5 μm. The duration of tackiness is preferably at least 10 seconds and usually less than 40 seconds and preferably less than 30 seconds, e.g., 10 to 30 seconds or 10 to 20 seconds. The period during which the coating can be cold sealed is usually somewhat less than the period of tackiness, but the period of tackiness is a useful guide to the sealing and blocking properties of the coating and for convenience is referred to herein as the deactivation period As a result of controlling the crystal usability and/or deactivation period, the invention makes it possible to obtain a better compromise between sealing performance and blocking problems. Thus, it is now possible to reduce the tendency for blocking problems to increase in proportion to sealing performance In particular we can achieve reduced blocking problems against most of the typical polar surfaces at any particular cold (and/or hot) sealing performance. Expressed alternatively, at any particular tendency towards blocking, it is now possible to obtain improved cold seal (and/or hot seal) performance The invention allows the conduct of fast commercial coating and sealing processes (for instance by selection of coating thickness, coating temperature, activation temperature and film speeds) that give an improved combination of good cold (and/or hot) sealing performance benefits with reduced blocking problems. Previously proposed heat activatable adhesive, thermoplastic polyurethane, coatings (and which we believe were made as described in CA-A-2, 174,288) have tended to give a large crystal size, typically well above 15 μm, and slow deactivation. This may lead to convenient cold sealing performance but unfortunately very high blocking tendency (with consequential difficulty in unwinding the roll of coated film) is inevitably a serious problem when making, converting and using these films in very high speed, compact, manufacturing processes. For instance there may be sticking on rolls and winders In the invention, we adjust the composition of the adhesive coating so as to achieve much smaller crystal size, and in practice this is associated with faster crystallisation and shorter duration of deactivation. As explained below, there are various ways of modifying the crystallisability of the coating composition so as to achieve the small crystal size. Best results for both hot and cold sealing applications are achieved when the crystal size is below 5 μm and generally when it is below 3 μm The preferred coatings have a crystal size of not more than 2 μm. Crystal sizes as low as this are associated with a significantly reduced blocking tendency with the result that the film or other sheet material can be coated on a high speed compact production line with minimum risk of blocking. If the crystal size is extremely small, this is associated with a rapid loss in tack and activity, with the result that it may be difficult to use the composition satisfactorily for cold sealing applications Generally therefore the crystal size is at least 0.3 or 0.5 μm. Best results for cold sealing applications seem to be achieved with a crystal size in the range 0.7 to 3, preferably 0.7 to 2 μm. However the very low crystal sizes, for instance down to 0 1 or 0.3 μm, can be used for heat sealing applications, depending on the choice of wax or other way of controlling crystal size. The thermoplastic polyurethane preferably has hydrophilic groups which render it water- dispersible. Preferably the polyurethane, and the total composition of coating solids and the coating itself, each have a melting point of from 40 to 100°C, generally 40 to 70° or 80°C. Best results are achieved with a melting point of around 50 to 70°C If the melting point is too low, there is increased tendency towards blocking The polyurethane, and the coating solids of the coating, preferably has an enthalpy of fusion in the defined melting range of at least 20 J/G and often at least 50 J/G, for instance 60 to 100 J/G. The polyurethane must be one which crystallises on solidifying from the molten or tacky state. Preferably it is formed from reaction of polyfunctional isocyanate and a mixture of high molecular weight (usually M w 500 to 5000 or higher) polyol and low molecular weight (usually M w 60 to 500) polyol, the polyols often being diols The high molecular weight diol or other polyol preferably has a melting point of from 30 to 100°C and an enthalpy of fusion in this temperature range of at least 50 J/G. Additional monomers that may be incorporated for forming the polyurethane are other hydroxy or amino compounds and other monomers which will introduce potentially hydrophilic groups so as to render the polyurethane water dispersible Suitable materials and methods for manufacturing the polyurethanes are described in CA- A-2, 174,288 by Licht et al and which is incorporated herein by reference. The molecular weight of the polyurethane may be selected to any chosen value (typically in the range 10,000 to 100,000) by selecting monomers and their amounts, and the polymerisation conditions, in conventional manner The coating solids (i.e., the total materials in the actual coating) consist mainly of the described polyurethane. Thus generally at least 60%, and normally at least 80% and generally at least 90% by weight of the coating solids are such a polyurethane Other components that can be present include thickeners, thixotropes, colourants, dyes and pigments together with fillers and blowing agents or any other conventional additives. The additives are preferably such that the coating remains optically clear. Preferred emulsifiers are aryl sulphonates. The preferred amounts are 0.05 to 10% by weight based on solids, generally 0.1 to 3% Preferably antifoam is present Suitable antifoams are silicone antifoams, typically added in amounts of 1 to 50, typically 2 to 10, parts per million, based on the aqueous dispersion. Preferably a slip aid is included in the coating composition so as to reduce the coefficient of friction coating/metal plate (COF) and to improve slip behaviour on the coating machines and on the packaging machines. These slip additives can be any of the conventional mineral or synthetic particles known for this purpose, generally having a size of 0.1 to 10 μm. Suitable materials include silicones, polymethyl methacrylates and various other minerals. The total amount of slip aid is generally in the range 0.1 to 5% by weight of coating solids, most preferably 0.3 to 1%) by weight of coating solids Preferably the coating has COF below 0.25, preferably 0.1 to 0.2, and if the coating without slip aid has a higher COF it is preferred to include sufficient of slip aid to reduce the COF to the desired value The total amount of these minor additives is usually below 20% and preferably below 10%, e.g., 1 to 5 or 1 to 10% by weight of the coating solids The coating solids are formulated so as to provide a coating having the desired crystallisability and/or deactivation period. One way of achieving this is by appropriate selection of the polyurethane component. Conventional reproduction of the manufacturing processes described in CA-A-2, 174,288 will tend to give a polyurethane having an essentially monomodal molecular weight distribution. Reduced crystalline size and faster deactivation can be achieved by use of a multimodal distribution, achieved by blending lower and higher molecular weight heat-activatable polyurethanes. Thus at least 10% and generally at least 30% by weight of the polyurethane can be a relatively low molecular weight polyurethane, for instance having a substantially monomodal distribution of from 10,000 to 45,000, often around 20,000 to 30,000, and at least 10% and generally at least 30% by weight of the polyurethane has a higher molecular weight distribution, for instance being a monomodal molecular weight distribution with an average of from 50,000 to 100,000, often around 50,000 to 70,000 Preferably the proportions of low and high molecular weight polyurethanes are 2: 1 to 1 :2, generally around 1 : 1 by weight. If desired, three or more polyurethanes may be blended. All molecular weights herein are determined by gel permeation chromatography against polymethyl methacrylate standards. Instead of using a multimodal distribution, we usually prefer to select polymerisation materials and conditions such that the polyurethane has an essentially monomodal distribution with an average molecular weight of at least 10,000 and usually at least 20,000 and most usually at least 30,000, but usually not more than 80,000 or, at most, 100,000 We find best results are generally obtained in the range 30,000 to 60,000. There is no indication in CA-A-2, 174,288 of the effect of molecular weight, but we find it influences performance. As a generality, cold sealing performance and blocking problems both increase with increasing molecular weight, and decrease with decreasing molecular weight. The best combination of good sealing with low blocking is generally achieved within the quoted ranges, preferably 30,000 to 60,000 There can sometimes be a tendency for some reduction in molecular weight during storage, by hydrolysis, and so it is preferred to make the polymer to a slightly higher molecular weight than is finally required for optimum performance. The monomodal molecular weight distributions used in the invention generally reflect a M w /TVI n ratio of between 15 and 1, generally between 5 and 1. With a monomodal distribution the preferred crystal size and/or deactivation period generally is not obtained unless a crystallisation initiator is included in the coating solids. Accordingly, irrespective of the molecular weight or molecular weight distribution, the preferred coating solids in the invention include a crystallisation initiator whereby the size of the crystals is reduced relative to the same coating solids underthe same test conditions but formed in the absence of the crystallisation initiator Instead of referring to the additive as a crystallisation initiator, it may be referred to as a nucleating agent Crystallisation depends upon the interaction between the polymer and the additive or additives in the coating solids which promote crystallisation As regards the polymer, it should be one which has a molecular structure which permits crystalline ordering and which can crystallise at a temperature below the melting point but preferably not too close to the glass point. As regards the crystallisation initiator or nucleating agent, this additive should be a material which has a melting point higher than the melting point of the polymer, which is insoluble in the polymer but which is wetted or absorbed by the polymer and which can be homogeneously dispersed in a melt of the coating solids as a very fine dispersion. The overall effect of the additive should be to increase the isothermal recrystallisation velocity of the composition from a molten state and which will promote a spherulitic crystalline superstructure and reduce the crystal size to the desired size, as a consequence of this. Nucleating agents for crystallisable melts are well known and materials suitable for any particular coating composition or coating can be found by routine testing. Mixtures of materials may be used to provide the nucleation or crystallisation effect which is required in the invention. Indeed, some suitable additives are themselves mixtures of materials, for instance various waxes. Preferred waxes are hydrocarbon waxes which melt at a higher temperature than the polyurethane, and which preferably melt at above 80°C and generally above 100°C, for instance at 100 to 150°C. Thus although paraffin waxes having a melting point of, for instance, above 50°C can be suitable with some polymers, it is generally preferred to use higher melting waxes such as waxes having melting points of 50 to 150°C, preferably 80 to 130°C The waxes can be cyclic or branched hydrocarbon waxes but are preferably substantially linear, aliphatic, saturated waxes. Fischer Tropsch waxes are particularly suitable. Waxes which are based on aliphatic and/or aromatic esters or mixtures of these optionally with fatty acids and/or fatty alcohols, ketones or hydrocarbons can also be used. Suitable ester waxes are Carnauba and, especially, Montan wax. It is particularly useful to use waxes since they can migrate to the surface and provide further reduction in blocking (in addition to the reduction in crystal size and/or deactivation period), and in COF, by a surface effect Other materials from which suitable nucleating agents can be selected from (depending upon the particular polymer) conventional inorganic nucleating agents such as kaolin, silica and talc and conventional organic nucleating agents such as salts of acids (for instance lithium benzoate) aryl sulphonates and certain pigments, as well as waxes and particular polymers such as ethylene/acrylic acid copolymers The amount of the crystallisation initiator will be selected so as to promote the desired crystal size. It is usually at least 0 1% by weight of the solids and generally at least 0.5%) It is usually below 20% and usually below 7% by weight of the coating solids. Best results are generally obtained with from 0.5 to 5, generally 0 5 to 2.5, parts crystallisation initiator per 100 parts coating solids Since the coating solids are mainly polyurethane, the amount can alternatively be expressed as 0 5 to 5, preferably 0 5 to 2 5, parts by weight initiator per hundred parts polyurethane. The coating composition should be formulated so that it does not interfere with the optical properties of the coating or the coated film. In practice this usually requires that the coating should be substantially clear and transparent, and so the selection of the initiator and the other components of the coating is preferably made so as to minimise any tendency towards opacity or translucency in the coating when it has crystallised The reduction in crystal size achieved in the invention is beneficial in this respect since the transparency of the coating will generally tend to improve as the crystal size is reduced, with the coatings having very small crystal size being substantially wholly transparent. This is of particular benefit when the sheet substrate which is coated is itself transparent. The coated sheets of the invention are made by applying a coating composition containing the coating solids to the sheet The method of production is preferably continuous and thus the preferred processes of the invention comprise continuously forming a coating by applying the coating solids to the film and then winding up the coated film or other sheet. The coating initially has to be in a liquid form in order that in can be applied and thus the coating initially goes through a tacky state. The process generally involves cooling the coating to render it non-tacky, and the coated film should therefore not be wound up until it is non-tacky and until the tendency to blocking is sufficiently low. The liquid form of the composition during its application to the sheet can be a melt, solution, emulsion or dispersion For instance the coating could be applied by a melt coating technique through a dye or other melt coater Preferably the composition is an aqueous composition, preferably an emulsion or dispersion of the coating solids in water. It may be applied by any convenient technique, preferably by a gravure roll such as in direct gravure technology. The coating solids preferably include emulsifier and antifoam, as discussed above, in order to facilitate the application of the coating. The total solids content of the aqueous dispersion or emulsion is generally in the range 10 to 60%, preferably 20 to 40%, by weight The coating is generally applied to the film or other sheet material by any conventional doctor blade, roller coating or other coating mechanism. The dry weight of the coating is usually at least 0.3, and preferably at least 0.5 g/m 2 . There is usually no need for it to be more than 5 g/m 2 and usually it does not have to be more than 3 g/m 2 A particular advantage of the invention is that good results are obtained at low coating weights of below 2g/m 2 , for instance 0.5 to 1.9 g/m 2 . Suitable coating thicknesses are from 0 5 to 2 μm The coating is fluid when it contacts the film and it is usually necessary to heat the coating in order to drive off the water or other liquid carrier Thus the film is usually either coated in an oven or passes through an oven substantially immediately after coating, the oven being at a suitable temperature to drive off the water or other liquid carrier. Typically the oven is at a temperature of 50 to 100°C or more, usually at a temperature of above 90°C. The heating temperature is usually above the melting point of the coating solids and the coating. The coating procedure results in the formation of a tacky coating. In the invention, the coating is then cooled so as to render it non-tacky. The cooling can be by mere exposure to ambient conditions but preferably the coated film is chilled by a chill roll and/or by a forced air stream at the desired cooling temperature so as to accelerate the cooling Thus air at, for instance, 5 to 30°C, often 25°C, can be blown across the tacky coating Accelerating the cooling and/or providing a low coating weight can both contribute to the formation of the desired small crystal size, irrespective of the choice of nucleating agent or other combination of materials to promote crystallisation. The cooling must not only render the surface non-tacky to the touch, but must also result in the coating having a suitably low tendency to blocking when it is wound up at the end of the coating operation. Typically the duration between the start of cooling and the winding is from 1 to 20 seconds, most preferably 3 to 10 seconds. At the time of applying the coating, the sheet is generally travelling past the coating station and to the winder at a speed of from 200 to 1000 metres per minute, preferably 250 to 600 metres per minute. The coating can be a continuous coating, i e , an overall coating, or it can be discontinuous. For instance the coating can be applied only to patches, strips or other regions where adhesion will be required. The coating weights mentioned above relate to the weight per square metre that is actually coated. The sheet material can be any sheet which is to be bonded by hot or cold sealing to another surface. The sheet can be, for instance, a paper sheet cardboard or metal foil but preferably the sheet material is a polymeric film The preferred film is a polyolefin film such as a film based on polyethylene, polypropylene, polybutylene, or a copolymer or terpolymer of ethylene and or propylene and/or butylene, with polyethylene or polypropylene being the preferred film. This film may be constituted by several layers of different polymers which are coextrdued, the main layer in weight, being polypropylene or polyethylene. By this way at least one external side of the film could be constituted by polyethylene, polypropylene, copolymer PE-PP, terpolymer PE-PP-polybutylene, copolymer ethylene-vinyl alcohol, or any other type of extrduable polymer. Other polymeric films include PET or other polyester films, polyamide films, polyacrylonitrile films, cellulose derivative films, polyacrylate films, polystyrene and other polyaromatic films and polyvinyl films such as films based on polyvinyl chloride, polyvinylidene chloride and polyvinyl acetate. Films based on copolymers may be used. The films may have been made by conventional film-forming techniques to a thickness of, typically, 3 μm to 5,000 μm, preferably 10 μm to 200 μm The films may be made extrusion, coextrusion, coating or lamination. The films can consist of a single layer or a laminate. They may be unoriented or they may be monoaxially or biaxially oriented. They may be cavitated (for instance as a result of biaxial orientation of a film containing a cavity-formation component such as spheres of polybutyl terephthalate as described in US patents No 4,632,869 and 4,720,416, or any other type of particles including some cavities) The films may contain conventional additives such as dyes, pigments, antistatic additives and stabilisers. The surface of the film on to which the polyurethane coating is to be applied may carry a suitable primer in order to improve adhesion, for instance if the film would otherwise give poor adhesion. Suitable primers depend upon the composition of the coating solids and the nature of the film and are selected in conventional manner from conventional primers such as polyimines, polyurethanes, polyesters, polyethers, polyacrylates, chlorinated polypropylene and epoxy primers. The adhesion of the primer to the film, or the adhesion of the polyurethane coating to the film, can be improved by for instance corona, flame or plasma treatment of the film surface if desired. It is naturally necessary to select the polyurethane, the film substrate, and the primer (if present) in conventional manner so as to obtain appropriate coating and bonding. Particularly preferred films according to the invention are polypropylene films, and in particular such films carrying an epoxy primer beneath the polyurethane coating If desired, the polypropylene film may comprise a polypropylene core with one or more coextruded facing layers. The core may be a microvoided core The reverse face of the film can be non-polar but the invention is of particular value when the reverse face of the film is polar (for instance so that the film is printable). There would (prior to the invention) then be a particular tendency for the polyurethane to block against the polar reverse face of the film when the coated film is wound up The polar properties can be provided by a primer or by flame, corona, plasma, electron beam or chemical grafting. In preferred films there is a hydrophilic acrylic coating on the reverse face of the film, for instance an acrylic coating. Another useful embodiment of the invention arises when the reverse face of the film is coated with a material to which significant blocking does not occur in the roll but to which heat sealing can be effected by the application of appropriate heat and pressure at a subsequent stage. For instance the reverse face can be coated with an acrylic coating composition or with a polyvinylidene chloride coating composition. It is also possible to improve the barrier properties of the film by providing a silane, aluminium or other metal, or silicate, alumina or other oxide coating on the reverse face. Thus the coated film may have the crystallisable polyurethane coating on one face and the other face may be metallised with aluminium or otherwise treated with silane, oxide or other metal. The invention also includes processes where the crystalline polyurethane is applied over a preliminary coating of metal, oxide or silane or other barrier-improving substrate for the polyurethane. The film or other sheet carrying the adhesive polyurethane coating is laminated to a receiving sheet by the adhesive coating, either by hot sealing or by cold sealing. The sheet material which is to serve as the receiving sheet may be formed from any of the films or sheet materials discussed above as the substrate for the adhesive polyurethane coating. Often the surface of the receiving sheet material to which the adhesive coating is to be bonded is itself a coating on a substrate, for instance a coating of polyvinylidene chloride, acrylic polymer, polyvinyl alcohol, epoxy, or polyurethane Often the coated sheet material is to be bonded to another piece of the same material or it may be bonded to itself to form a package Thus the coated sheet may be folded upon itself and then end sealed and/or edge sealed either with the polyurethane-coated face bonded to the reverse face of the film to form a flat seal or two polyurethane-coated surfaces may be bonded in face-to- face contact to each other as a fin seal, both polyurethane coatings preferably being in accordance with the invention. The bonding may be achieved by activation of the coating followed by sealing, or by simultaneous activation and sealing. Usually it is preferred to preactivate the coating. The preactivation can be by conduction, convection or radiation. For instance it can be by infra-red radiation, hot air or through contact with a heated surface Activation by methods that do not require contact between a heated surface and the film is generally preferred, for instance hot air or, preferably, infra-red radiation The coating on the film, during the activation, is generally heated to close to or above its melting temperature. The activation must be sufficiently intense to render the coating tacky and usually the maximum surface temperature of the coating during activation is from 50 to 160°C, preferably 80 to 130°C. The entire coating may be activated if required, but it is often preferred to activate only those parts of the coating which are to be sealed, for instance in a pattern. Thus preferred films according to the invention have an overall heat-activatable coating but are then activated only in those regions where sealing is to occur For instance localised activation in this manner can be achieved by localised direct or indirect heating of the coating in the desired pattern of activation. Sealing is applied after activation by pressing together by jaws the surfaces which are to be bonded to one another. The duration between activation and the sealing step is normally such that the coating still feels tacky at the time it is sealed and so is usually less than the measured activation period for the coating. Generally therefore the period is from 3 to 30 or 40 seconds. Because of the short deactivation period, the jaws or other sealing apparatus can be located relatively close to the activation stage, even when cold sealing is to be used, and thus the packaging machinery can be very compact For instance the distance between the activation stage and the sealing stage is typically in the range 1 to 5 metres and the packaging speed is typically in the range 9 to 120 metres per minute. When the invention is applied to forming a package around packaged goods, the preactivation is generally applied prior to the forming collar of the packaging machine, and the sealing is applied subsequently by appropriate jaws In preferred processes of the invention, activated coating is cold sealed to the receiving surface while it is still activated. The temperature of the sealing jaws is generally at about or below ambient (for instance 0 to 30°C) but if desired the jaws may be heated slightly for instance up to 35°C or 40°C, or other cool temperature well below the melting point of the coating. In other processes of the invention, the activated coating is hot sealed by the application of heated jaws, for instance having a temperature of less than 100°C, generally 40 to 80°C. When, as is particularly preferred, the invention is applied to the packaging of heat- sensitive components such as chocolate, ice cream or other foodstuffs, the coating is activated (usually before the forming collar) and then, at a position sufficiently distant from the activation heaters, the heat-sensitive component is wrapped in the activated sheet material to form a packet, and the sides and/or ends of the packet are then cold-sealed between cold jaws or other cold sealing apparatus. In such a process, preferably the activation is confined solely to those areas where cold sealing is to be applied If desired, there can be a cold air curtain or other cooling apparatus between the means for activation and the position at which the heat-sensitive goods are introduced into the package that is being formed, so as to protect the goods. When cold sealing (including cool sealing) is required, it is particularly preferred to bond together two coatings of the invention, for instance as a fin seal. Preferably the invention is used for cold sealing in which event a preferred process comprises heat activating the heat-activatable coating on the sheet or sheets and subsequently cold-sealing or cool-sealing the coating while still activated. The invention can also be applied to substantially simultaneous activation and sealing processes in which the film is activated either by the sealing jaws or immediately ahead of the sealing jaws, i.e., with the activation heat and the sealing pressure being applied substantially simultaneously. Thus the bonding may be by heat sealing, in which event the sheets which are to be bonded to one another may be pressed together between heated jaws, i.e., appropriate other heated pressure applicators in conventional manner so as to provide melt sealing. The jaws may be heated to above the meeting point of the coating, but an advantage of the invebtion is that cool sealing at for instance above 40°C, and often above 50°C up to 70°C or 80°C can give good results. Suitable sealing machines are Horizontal Form Fill Seal (HFFS), Vertical FFS (VFFS), or other suitable machines. Instead of using the invention for sealing packages, it can also be applied for lamination of two sheet materials, for instance a film or other sheet coated with the crystallisable polyurethane of the invention and a receiving sheet which may be, for instance, board, film or other sheet which may be coated with crystallisable polyurethane or may be coated with other coating which will promote sealing (generally heat sealing) or may be uncoated. The lamination may involve bonding over the entire interface between the two sheets (in which event a continuous coating of the polyurethane would be required) or it may merely involve localised bonding, for instance in stripes or other pattern. The bonding used for lamination can involve cold sealing but it is usually convenient to conduct it by hot sealing either between two hot-heated rolls or between one cold roll and one hot roll, the latter generally being the roll which drives the polyurethane-coated film. When hot- sealing by this or other means is to be conducted the temperature of the heated roll or rolls should be above 50°C and preferably above 80°C The crystal size is determined in the invention by examining the relevant film using an optical microscope to observe crystal sizes where most of the crystals are above 2 or 3 μm, and using an Atomic Force Microscope to determine smaller crystal sizes. In each instance the operator selects a random area (or several random areas if there is significant variation between them) and measures the horizontal and vertical dimensions for a number of the crystals or spherulites that are observed, and thereby calculates the diameter of each spherulite. A typical selection of a plurality of these spherulites is made and the average diameter is calculated from each of these. In practice, we usually find that at least 50% (by number) of the crystals have a size close to the average. If it is found that there is a wide spread in diameters within one sample or from one sample to another, then sufficient samples are examined in order to give a reasonably accurate value for the average diameter. In order to measure the deactivation period, a coated sheet is cut inwards from one edge into a series of parallel bands each 30 mm wide After the sheet is heated for one minute at 100°C in an oven in order to render the coating molten and tacky, the sheet is removed from the oven and exposed to ambient temperature. At 5 seconds, the first of the bands is pressed on itself and immediately peeled apart by hand. This is repeated at 10 seconds for the second band, 15 seconds for the third band and so forth. The last coating which shows significant tackiness when peeling the band away from itself indicates the time for which the coating remains tacky. Thus if the band at 30 seconds does feel tacky but the band at 35 seconds does not, the coating is assessed as remaining tacky for 30 seconds and as having a deactivation period of 30 seconds. In each of these tests, a useful guide as to the potential suitability of any particular coating composition is the performance in each test of a sample film made by coating lg/m 2 of the composition on an epoxy-primed polypropylene film. The following are examples of the invention EXAMPLE 1 A polyurethane as described in CA-A-2, 174,288 is emulsified in water to give an emulsion having a 20% solids content together with 0.1% aryl sulphonate as emulsifier, 10 ppm of a silicone antifoam, and 0.5 parts (per hundred parts polyurethane) of a slip aid which is sold under the trade name Tospearl and which consists mainly of spherical cross linked particles having an average diameter 4.5 μm. The coating composition is made by blending these ingredients using a pneumatic stirrer. The inclusion of the slip aid had the result that the coefficient of friction (COF) of films made from the coating, as discussed in the following examples, generally fell within the range 0.13 to 0.25, whereas in the absence of sufficient (or any) slip aid the typical COF for the composition is around 0 4 or higher During the preparation of the composition, either a mixture of polyurethanes was used or an addition of crystallisation initiator was added in many of the processes, as set out in the table shown below. The polyurethane had M w /M„ between 5 and 1 and had a weight average molecular weight of 50,000 (determined by GPC against polymethyl methacrylate) unless otherwise stated. EXAMPLE 2 The aqueous dispersion of example 1 was coated by a direct gravure process and the coated film was heated to 110°C so as to dry it and convert the polyurethane coating into an adhesive coating and was then cooled by air. The crystal size in the resultant coating was determined by Atomic Force microscope or (for the larger crystal sizes) by optical microscope, as explained above. In some examples, the activation period (the time taken for the coating to be cooled down during manufacture from its temperature in the coating oven to the time when it no longer exhibited tacl , as explained above). In order to test blocking, the polyurethane coated surface of the film was pressed against the reverse surface of the film 52.5 kg/cm 2 pressure was applied for 24 hours at 4°C in order to simulate what can happen in a roll The films were then peeled apart and the peeling force was recorded as grams per 25 mm. Thus the blocking value is for the A/B configuration. Cold sealing was conducted using a horizontal form fill seal (HFFS) packaging machine operating at various web speeds, usually 50 metres per minute or 80 metres per minute. The jaws were at a temperature of 20°C and were located at a travel time of 1.8 seconds when the film is travelling at 50 m/min and 1.1 seconds when the film is travelling at 80 m/min, i.e., a linear distance of 1.5 metres from the activation stage which used short wave infra-red and generated a surface temperature of 60°C In each instance the polyurethane coating was being pressed against, and sealed to, itself and thus the HFFS cold value is for the A/A configuration. In other tests, the film was not pre-activated but instead was subjected to heat sealing using heated jaws. In one of these tests the heat seal was determined as an HFFS value at a draw temperature of 80°C and a film speed of 40 m/min for the A/A configuration. In another test, the minimum sealing temperature which was required to reach a sealing strength of 250 g/25 mm for the A/A configuration was recorded The results are set out in the following tables. The results in Table 1 are the results obtained when the starting film is a 33 μm white cavitated polypropylene film having a density of 0 6 having coextruded surface layers of terpolymer of propylene, ethylene and butylene (Terpo) and which is reverse coated with an acrylic coating and coated on the other face with an epoxy primer. The polyurethane coating is then applied on to this primer. F/T wax is Fisher Tropsch wax. TABLE 1 These results show that the low molecular weight polyurethane gives a satisfactorily short activation period and reasonable blocking performance but poor sealing performance, whilst the higher molecular weight polyurethanes, when used alone, give too slow an activation period and bad blocking performance (and also large crystal size) but improved sealing performance, both cold and hot. Test 4 shows that a better compromise between blocking and sealing is obtained by blending high and low molecular weight polyurethanes. Test 5 shows that an improved combination of blocking and sealing can also be obtained by adding 2.5 phr (parts per hundred resin) paraffin wax. The addition of montan wax gives a further improvement in blocking but with a slightly adverse effect on sealing properties and so if montan wax is used the amount may need to be optimised depending upon whether the primary objective is improved sealing or reduced blocking In this particular test carnauba wax was found to prevent cold sealing although it gave reasonable hot seal and good blocking properties. This cold sealing failure may have been due to unknown additive or other component in that particular sample of in the carnauba wax. Test 6 shows that the best combination of blocking and sealing properties, combined with the smallest crystal size and a short activation period, is obtained using 2.5 parts Fischer Tropsch wax per hundred parts resin, as nucleating agent. The results in Table 2 show the effect of varying the nature of the base film (the film itself, its surface coating, its primer coating or its reverse surface) when applying a system otherwise similar to test 6, i.e., using polyurethane molecular weight 50,000 with 2.5 phr Fischer Tropsch wax. In Table 2, film A is the 33 micron polypropylene cavitated white film having density 0.6. Film B is a 35 μm polypropylene white cavitated film having a density of 0.72. Film C is a 23 micron transparent plain polypropylene film having a density of 0.9 Film PET is a 12 μm plain transparent polyester film. Film OPA is a 12 μm plain transparent polyamide film and the paper is an 80 μm sheet. In each instance the primer is applied over a surface coating and in each instance the reverse surface of the film had an acrylic coating or other specified treatment, except as shown. TABLE 2 Comparison of tests 6, 9 and 10 shows that, as with any film coating process, selection of the primer can have negative influence on the final properties of the film and that it is therefore necessary to optimise the primer with respect to the substrate and the heat-activatable composition, in accordance with conventional techniques. Comparison of tests 6 and 11 show that even varying the density of the substrate film can influence results. Tests 12 to 22 show that useful combinations of sealing and blocking can be obtained with a wide variety of substrates, including variations in the reverse face treatments of the substrates. The consistent trend throughout all these results is that, for any particular combination of primed substrate that gives reasonable sealing properties when the thermoplastic polyurethane is applied, modification of the thermoplastic polyurethane coating in accordance with the invention will give an improved combination of blocking and sealing properties.

Claims: CLAIMS; I. A coated sheet having a heat-activatable adhesive coating of coating solids, wherein the coating solids are mainly thermoplastic polyurethane, the coating solids having a melting point of 40 to 100°C, and the coating is crystalline and has an average crystal size below 10 μm. 2. A sheet according to claim 1 in which the coating, when molten, remains tacky, when pressed on itself, for at least 2 seconds but not more than 50 seconds upon cooling at 25°C. 3. A sheet according to claim 1 or 2, wherein the coating solids include crystallisation initiator. 4. A sheet according to claim 3, wherein the initiator is a wax. 5. A sheet according to claim 3, wherein the initiator is selected from the group consisting of paraffin wax, Montan wax, and Fischer Tropsch wax. 6. A sheet according to claim 1 or 2, wherein the polyurethane has a substantially monomodal molecular weight distribution with an average molecular weight from 20,000 to 100,000. 7. A sheet according to claim 1 or 2, wherein the polyurethane has a multimodal molecular weight distribution wherein at least 30% of the polyurethane has a substantially monomodal molecular weight distribution with an average molecular of from 10,000 to 45,000 and at least 30% by weight of the polyurethane has a substantially monomodal molecular weight distribution with an average molecular weight of 50,000 to 100,000. 8. A sheet according to claim 1 or 2, wherein the coating is on one surface of the film and the other surface is a polar surface. 9. A sheet according to claim 1 or 2, wherein the sheet comprises a polypropylene film which has an epoxy primer on one surface and has been rendered polar on the other surface and in which a heat-activatable adhesive coating of coating solids comprising at least 90% by weight polyurethane and at least 1% by weight Fischer Tropsch wax and having a melting point of 40 to 100°C is coated on the epoxy primer. 10. A sheet according to claim 1 or 2, wherein the polymeric film is a microvoided polypropylene film. I I. A product which is a laminate of a first sheet according to claim 1 or 2 bonded to a receiving sheet by the adhesive coating 12. A process of making a coated film according to claim 1 or 2 comprising continuously forming a tacky coating by applying the coating solids to the film and cooling the coating until it is non-tacky and then winding up the coating 13. A process of bonding a sheet according to claim 1 or 2 to another surface by the heat- activatable adhesive coating comprising either heat-activating the coating and then cold or cool sealing the activated coating to the other surface, or by heat sealing the activated coating to the other surface.

Inventors: GRINGOIRE BRUNO R L, KELLER LAJOS E, LEGER CHRISTIAN, GRINGOIRE, BRUNO, R., L., KELLER, LAJOS, E., LEGER, CHRISTIAN
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1999
Main classification: C09J7
IPC codes: B32B27, C08G18, C08J7, C09J7, C09J175