Abstract:
The invention consists of a multilayer material and a process for manufacturing this material. The multilayer material comprises a polyester substrate which is provided with a thermoset plastic polymer coating and a layer of contact adhesive. The polymer coating is produced by a reaction between a polyurethane polymer containing hydroxyl groups and a low molecular linear polyisocyanate, with catalytic tin-organic additives and colorants. The multilayer material is resistant to tearing and is also weather-resistant. A transparent, reactive layer, consisting, for example of a layer to take toner or ink, or a metallic layer, can be deposited on the side opposite to the polymer-coated substrate. Multilayer materials of this kind are suitable for large-scale covering of supporting frameworks in the construction of model aircraft and are also suitable as a medium onto which images and print may be applied using a plain paper copier, and can also be used to carry advertising.

Description:
FIELD OF THE INVENTION 
     The invention refers to a multilayer material, consisting of a heat-activated substrate with a defined shrinkability on a polyester base, coated with a polymer and, if necessary, with a thermoplastic contact adhesive and, if necessary, coated with a layer for receiving toner or ink, or with a metallic coating. 
     FIELD OF THE INVENTION 
     Multilayer materials of this kind are suitable for covering large areas over a supporting framework in the construction of model aircraft and of one-man aircraft/ultralight aircraft. However, they are also suitable as a medium onto which images and print may be applied using a plain paper copier, and are used as weather-resistant, self-adhesive labels and marking materials and in advertising. 
     Because of their physical and chemical properties, coloured polyester films, made for example from polyethylene terephthalate are suitable for covering large areas on model aircraft, and are well known in this field. Due to their weather-resistant properties and resistance to heat, such films, when coated on one side with a layer for receiving toner or ink, can be used as a medium onto which images or print can be applied using a plain paper-copier. 
     Normal technical solutions involve coating transparent polyester substrates with a heat-activated, pigmented and/or coloured polymer adhesive layer. In addition there are polyester substrates in which a pigmented, thermoset polymer layer is bonded with the substrate by means of a thermoplastic polymer layer, with another thermoplastic polymer layer applied as an adhesive layer. Such films tend to be relatively heavy. 
     Fibrous supporting materials woven from polyester and impregnated with resin are also used in model aircraft construction to cover the wooden framework. The resinous solvents that are used can be coloured with pigments and/or organic colorants. 
     One disadvantage of these common technical solutions is, for example, the noticeable decline in the adhesive strength of those materials which are coloured by the application of pigmented adhesives. Moreover, the visual impact of, the covering material produced in this way depends largely on the conditions under which it is used and processed, thus making it impossible to guarantee consistent quality. Because of the thermoplasticity of such adhesives, when material is stretched over corners and edges cracks may occur in the pigment, with splits at the corners of the coloured/adhesive layer. Accidental contact with the adhesive layer and/or when adjustments are made to the covering material may spoil the continuity of the pigment structure. 
     Minute holes may be formed in the pigment layer of polyester films if their thermoset layer of pigment is embedded between two thermoplastic polymer layers, i.e. between the adhesive and its base. These are caused by a reaction in polyurethane systems between the hardener, such as isocyanate, and the thermoplastic adhesive layer. The reaction process releases gases (e.g. CO 2 , water), which then affect the pigment layer, forming minute holes. 
     Layers of toner are usually formed by applying bicyclical alkyl-, 1-12-C aliphatic alkyl or aromati-(meth)-acrylate with up to 20% of a polar monomer or N,N-dialkyl-mono-alkylamino-alkyl-(meth)-acrylate. Solutions of polymerised diol-di-(meth)-acrylate are also used, with additives consisting of copolymerised vinyl monomer and copolymerised vinyl ester, (meth)-acrylic esters and/or styrol. Transparent, aqueous layers that can accept toners and are made of water-soluble polymers, with polymer particles measuring between 1 and 15μ and with additives consisting of antistatic agents are also frequently used. Toner fixing layers are also used, made from polyvinylacetal resin with a degree of acetalisation of between 20-40 mol %. Furthermore, well known are water-soluble cellulose polymers containing non-ionic or ionic materials which are suitable for the production of transparent toner layers on polyester, e.g. Melinex or Hostaphan film. 
     And finally, clear polyester films with layers that can accept ink are also used, and are made up of vinyl, pyrrolidone particles of a polyester, a co-polymer of a 2-6-C alkylenoxide, a polyvinyl alcohol and inert particles. Current technology also means that viable films can also be made from polyethylene, polypropylene and polycarbonate, coated on one side to set the ink, and that this coating consists of emulsions/dispersions of vinyl chloride-vinyl acetate-copolymer resin. In general additives consisting of colloidal silicon oxide offer the best possible means of preventing the finished material from sticking. 
     A disadvantage of the films referred to above is that, depending on the type of application, and despite the fact that polyester films and non-woven materials are in principle suitable as the basic materials for weather-resistant coverings for model aircraft, and are resistant to tearing, such materials can only cover a limited area, and have a restrictive load-to-weight ratio. This means that they cannot be used for one-man aircraft. 
     Materials provided with the toner layers referred to above range from clear to opaque, and copies can be made in black on a transparent and/or opaque background. However, because of the transparency of the material they are of limited effectiveness or totally ineffective for advertising purposes. 
     SUMMARY OF THE INVENTION 
     The task of this invention is to improve the performance characteristics of heat-activated, coloured, shrinkable films on a polyethylene therephtalate base. Depending on the area of application of the film, in accordance with this invention, the film is polymer coated, firstly to enhance the surface finish of the thermoset pigmented film, to improve the tear-resistance of the film and to reduce the area weight, and secondly new areas of application are to be opened up by providing the film with a coloured backing, at the same time coating one side of the film with an impact adhesive. The purpose is also to coat the film in such a way that it can be used with copying systems. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A multilayer material and method of making same that overcomes the above disadvantages has now been discovered. In accordance with this invention, the multilayer material demonstrates improved performance characteristics with surprising effects when processed, as well as better resistance to tearing, and superior weather-resistance, compared with normal materials. In particular, as a result of so-called corona pre-treatment of the substrate, the film surface is modified in such a way (“roughened”) by chemical and physical methods that the bonding between the polymer coating and the substrate is substantially improved compared with normal methods. This is evident in the fact that the polymer coating is more effectively bonded to the film. However, this is subject to the precondition that the polymer coating process takes place immediately after the corona treatment. 
     The introduction of coloured pigments and/or fluorescent pigments and/or organic colorants to the reaction mixture, which consists of a hydroxy group polyurethane, preferably with hydroxyl groups in the final stage, and a low molecular weight linear polyisocyanate in a ratio of 20:1 with reference to solid polyurethane polymer, as the cross-linking agent, ensures a very homogenous coloration of the polymer layer. In accordance with this invention, a catalytic additive consisting of a tin-organic compound is used, reducing the polymerisation time to approximately 48 hours, compared with a period of several days if no catalyst is used. 
     The contact adhesive layer is applied using the well-known transfer method, in which contact adhesive materials are used on a polyacrylic basis in a commonly used composition. This is applied in solution to silicone paper, and after the solvent has evaporated it is bonded with the substrate under heat and pressure. 
     According to the invention, the side of the substrate opposite to the polymer coating is provided with a transparent reactive layer. This layer may be suitable for receiving toner or ink, and should preferably be of vinyl acetate, with the toner layer consisting of a substrate with a thickness of between 2 and 8 g/m. 2  This creates a polymer film with a compact or translucent coloured backing, which can be used with a plain paper copier to accept images and printing, without adversely affecting the quality of the text and/or images thus reproduced. Such films are also ideal for advertising purposes because, as mentioned previously, the pigment is distributed homogeneously throughout the polymer layer, and they are also very weather-resistant due to the thermoset properties of the film. 
     In order to keep the so-called blocking effect on the toner or ink compatible layers to a minimum, if necessary between 0.5 and 5% of a suitable agent for preventing this effect is dispersed. Such agents may consist, for example, of colloid silicon dioxide or alternatively—in the case of aqueous dispersions—of up to a maximum of 3% rice starch. The reactive layer may be light-sensitive, thus opening up other possible uses. 
     Moreover, in accordance with the invention, it is intended that the side of the film opposite to the polymer coating should be provided with a thin metallic layer, i.e. &lt;1μ, made preferably of aluminium and/or chromium. This enables the area weight of the film to be substantially reduced. By applying the metallic layer to the coloured, transparent film the latter appears coloured but not translucent. Compared with existing methods for producing such coloured films, the system referred to in this invention can enable weight reductions of between 25 and 30% to be achieved, while the chemical and physical properties remain unchanged. 
     Another task is to find a process for producing the multilayer material. In accordance with the invention, these measures have resulted in a process that ensures that the multilayer material maintains a consistent quality. In accordance with the invention, if necessary, a reactive layer is applied to the side of the substrate opposite the polymer and contact adhesive coating, consisting, for example, of a layer suitable for taking toner or ink, and that, if necessary, an agent can be dispersed in this layer to avoid a blocking effect on the film. In accordance with the invention the film is fed past ionising rods in order to avoid or reduce the electrostatic charge that is built up on the substrate through friction against the rolls, causing the static on the substrate to be discharged again. An alternative is to apply a metallic layer using the vacuum vaporisation method. 
    
    
     Other advantageous measures are described in the other sub-claims. The invention is presented using examples of the processes involved, and in the accompanying drawing, and is described in greater detail below. 
    
    
     A. 
     The single illustration shows the stages in the process in diagram form, in Appendix  10 . To apply the colour coating a substrate  11  made of polyethylene terephthalate is continuously unrolled from a reel  12 , and fed through guide rolls  13  and  14  to a unit  15  (a so-called corona discharge unit), containing a roll  16 . The substrate  11  is wound around this roll  16  in such a way that its surface is positioned facing the device  15   a , which is used to treat the surface of the substrate. The period of treatment is determined by the circumference and rate of rotation of the roll  16 . The corona discharge unit  15  can be incorporated in the process, i.e. if needed the unit can be connected up, according to the particular stage of the process. 
     Following corona treatment of the substrate  11  it continues over the guide rolls  17 ,  18  and  19  to a pair of rolls, consisting of a rubber roll  20  and a chromium plated steel roll  21 . The roll  21  serves as pick-up roll and rotates in a trough  23 . The (coloured) polymer mixture is contained in this trough  23 , and is applied to the pick-up roll  21  by a doctor roll fitted with doctor blades (not shown) to ensure that the correct amount is applied. 
     Once the film  24  has been coated (with pigment) it is fed through parallel pairs of ionising rods  25 ,  25   a  and  26 ,  26   a , to reduce or eliminate the electrostatic charge that is built up through the friction of the rolls. The film  11 ,  24  is fed over the system of rolls  27 ,  31  and  36  and through a drying tunnel, which is provided with an inlet  29  and outlet  30 , in which the solvent, for example toluol, is vaporised. Heated air is blown through the drying tunnel  28 , the incoming air  32  being heated to a specified temperature by the heating spirals  34  of a heating unit  33 . The exhaust air  35  is removed from the system. The films  11 ,  24  are dried using the counter-current process  32 ,  35 ,  48 . 
     The film is then fed through a system of rolls  38 ,  41 ,  42 ,  43  and  44  to a winder roll  47 , the roll  41  being fitted with a cooling device (not shown), to cool the heated film, thus preventing blocking of the films and/or preventing them from sticking together. The films are once again fed through parallel pairs of ionising rods  45 ,  45   a  and  46 ,  46   a  to reduce or eliminate the build up of an electrostatic charge. 
     B. 
     A slightly modified process in the same installation  10  is used to wind paper  11 , which is silicone coated on one side, from the reel  12 . This paper is then fed to the rubber roll  20 , where, in this process, the film  11  is coated with contact adhesive. For this purpose the trough is loaded with the appropriate adhesive mixture (not shown), which is transferred to the film  11  by means of the doctor roll  22  and pick-up roll  21 . The corona discharge unit is not used in this process. The amount of coating can be controlled or dosed by varying the pressure of the pick-up roll  21  on the rubber roll  20 . Analogous to process A, the coated film  11  is dried in the drying tunnel  28  and is then dispatched to a laminating device  37 , which is not connected in process A. The laminating device  37  consists of a steel roll  38 , which can be heated, and a hard rubber roll  39 . The film  11  is fed between these two rolls  38  and  39 . At the same time a web  40   a , coated in pigment in accordance with process A, runs through the laminating device  37  from an unwinding device  40 . The pressure of the rolls  38 ,  39  and the heat from the steel roll  38  causes the transfer of the adhesive from the silicone paper to the coloured film. The multilayer material produced in this way is then fed once again over the cooling roll  41 , and over the guide rolls  42 ,  43 ,  44  to the winding roll  47 . Analogous to process A, in this process the build up of static electricity is discharged by the ionising rods  25 ,  25   a ,  26 ,  26   a ,  45 ,  45   a ,  46 ,  46   a.    
     C. 
     Analogous to processes A and B, the trough  23  can also be supplied with a mixture which is suitable for the application of a reactive layer, for example one that is suitable for taking toner or ink. This alternative process is carried out immediately after the film  11 ,  24  is coated with adhesive. 
     D. 
     The film  11  can also be bonded with a non-woven polyester or with woven fabric in order to improve its mechanical properties, in particular the resistance to tearing. This alternative process takes place prior to the coating with the thermoset colour layer, in the same way as in process A. In this case the trough  23  is supplied with the adhesive mixture and, after the film  11 ,  24  has been coated with adhesive and dried, it is laminated with a non-woven or woven material  40   a  in the laminating device  37 . Coating with the thermoset colour layer then takes place. 
     E. 
     Analogous to the previous processes, the coloured, transparent polyethylene terephthlate film, which has a thickness of up to 10μ, is also provided with a thin aluminium layer (&lt;1μ), which is applied using the vacuum vaporisation process (not shown). The vaporisation should not take place until a hardening period of at least 48 hours has elapsed. 
     According to this invention, it should be possible for all the stages of the process to be performed in one single installation. These individual stages of the process can be carried out in some cases in an uninterrupted sequence. In other cases they have to be carried out successively, after the supply of materials has been changed. 
     PROCESS EXAMPLE 1 
     10-15 percentage weight of a polyurethane polymer with final hydroxyl groups with a hydroxyl value ≦5 are dissolved in toluol with a water content of &lt;0.1%. For coloration 12-15 per cent in weight of a coloured pigment such as titanium oxide, and/or a fluorescent pigment and/or an organic colorant are dispersed in this solution. Up to 1 percent in weight, calculated as a solution of a low-molecular weight polyisocyanate and up to a maximum of 0.05 per cent by weight of a tin organic compound are added to this solution as a catalyst. At a wet weight which, depending on the pigment, ranges between 12 and 30 g m 2 —in the case of titanium dioxide, for example, it is 30 g/m 2 —this colour solution is diluted using toluol in a DIN beaker using a 4 mm nozzle with a maximum run-off time of 90 sec. The solution is then applied mechanically, using standard roll applicator systems, to polyethylene therephthalate film (PETP film), which has been pre-treated in a corona discharge unit immediately beforehand. The PETP film has a defined shrinkage of approx. 5% both horizontally and vertically. The colour solution is dried at a maximum temperature of 80° C. to retain the shrinkability of the PETP film. The film is simultaneously set during this drying process. The quantity of dried colour coating amounts to between 15 and 30 g/m 2  of the PETP film, depending on the pigment. Following intermediate storage for 48 hours at room temperature, which should not exceed 35° C., a contact adhesive is applied at 21 g/m 2 ±1 g by means of transfer coating. The chosen adhesives are contact adhesives on the basis of polyacrylates in a commonly used composition. These are applied in solution to silicone paper, and after the solvent has evaporated they are bonded to the PETP film under heat and pressure. When the finished material has cooled the contact adhesive is bonded with the thermoset plastic colour coating and, until processing by the user takes place, this silicone paper serves to protect the applied coatings from scratches and from dust particles. 
     PROCESS EXAMPLE 2 
     The process takes place as in Example 1. The layer suitable for taking toner or ink is applied to the top side of the film immediately after it has been coated with contact adhesive. This is achieved using a solution of polyvinyl acetate in toluol, diluted further with toluol in a DIN beaker up to a run-off time of 16 sec., using a 4 mm nozzle. The solvent is evaporated off at a temperature of between 80 and 85° C. The quantity applied amounts to between 2 and 8 g by dry weight per m 2  of film. 
     PROCESS EXAMPLE 3 
     The process takes place as in Example 2, with between 0.5 and 5% colloidal silicon dioxide being added to the layer intended for the application of toner or ink, to prevent the blocking effect on the film. The silicon dioxide is introduced using a dispersion disc. 
     PROCESS EXAMPLE 4 
     The process described in example 1 is repeated, but in this case the PETP film is bonded with non-woven polyester prior to coating, in order to improve the mechanical properties, and in particular the tear-resistance. A 2-component adhesive on a polyurethane base, dissolved in toluol, is used to laminate the PETP film with the non-woven material. Thirty minutes before the solution is processed, it is mixed with 1 percent by weight, calculated on the basis of the polyurethane solution of a low molecular weight polyisocyanate, NCO content 11.5%. Evaporation of the solvent takes place at 80° C. Following storage for 48 hours under room conditions, the colour solution is applied to the substrate, followed by the application of the contact adhesive, as in Example 1. 
     PROCESS EXAMPLE 5 
     The process described in Example 1 is repeated, but instead of the contact adhesive, a heat-activated adhesive is applied to the polymer coating of the substrate. The heat-activated adhesive consists of a solution of linearly structured polyurethane polymer containing hydroxyl groups with a hydroxyl value of ≦5, in toluol. The dry coating of adhesive should amount to between 21 and 25% g/m 2  of the film. The solvent evaporates at between 80 and 100° C. As explained in Example 1, the transfer process is used to apply the heat-activated adhesive. 
     PROCESS EXAMPLE 6 
     The process described in Example 1 is repeated: a coloured, transparent PETP film (thickness 5-10μ) is provided with an aluminium layer (thickness&lt;1μ) using the vacuum vaporisation process. Films produced in this way are then provided with a heat-activated polyurethane adhesive, using the transfer process. The adhesive is produced by dissolving 12-15 per cent by weight of a polyurethane polymer with a hydroxyl value ≦5 in a mixture of toluol and thethyl ketone, in a ratio of 9:1. 
     The heat-activated polyurethane adhesive can also be replaced by a contact adhesive on a polyacrylate base, using a standard composition. 
     REFERENCE SYMBOLS 
       10  plant for producing a multilayer material 
       11  substrate 
       12  reel 
       13  guide roll 
       14  guide roll 
       15  corona discharge unit 
       15   a  device for surface treatment 
       16  guide roll for  15   
       17  guide roll 
       18  guide roll 
       19  guide roll 
       20  rubber roll 
       21  pick up roll 
       22  doctor roll with blade 
       23  trough for polymer coating 
       24  coated film 
       25  ionising rod 
       25   a  ionising rod 
       26  ionising rod 
       26   a  ionising rod 
       27  guide roll 
       28  drying tunnel 
       29  inlet aperture 
       30  outlet aperture 
       31  guide rolls for  28   
       32  incoming air 
       33  heating unit 
       34  heating coils 
       35  exhaust air 
       36  guide roll 
       37  laminating device 
       38  heated steel roll 
       39  rubber roll 
       40  roll with coated film web or non-woven material 
       40   a  coated film web or non-woven material 
       41  cooling roll 
       42  guide roll 
       43  guide roll 
       44  guide roll 
       45  ionising rod 
       45   a  ionising rod 
       46  ionising rod 
       46   a  ionising rod 
       47  winding reel for finished product 
       48  transport device