Abstract:
The method for making a multilayer composite having one or more colors brings together a number of acrylic layers, which are partially cured in a first step and completely cured in a second step. The curing takes place with actinic radiation, such as accelerated electrons, UV radiation or X-ray radiation, the curing unit operating with different dosage rates during the two steps. The curable acrylic layers are applied to the respective supporting layers by screen printing or stencil printing, or else may be applied to the supporting layers by casting or with the aid of printing rollers.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method for making a colored multilayer composite by laminating to each other, and curing, two or more radiation-curable layers, one of these layers being a clear outer layer and the other layers being equipped with color pigments, and also to a colored multilayer composite of this type, produced by the method.  
         BACKGROUND OF THE INVENTION  
         [0002]    WO 94/09983 has disclosed that colored vehicle parts can be produced with at least two different shades by transfer onto the vehicle parts of colored acrylic layers which have been applied to casting films. A laminate of this type is composed of a first polyester supporting layer, of a clear layer made from an optically clear polymer which comprises fluorinated hydrocarbon resin and acrylic resin, the clear layer having been applied on the surface of the supporting layer, and also of a binder layer and of a color layer made from chlorinated polymer with dispersed color pigments. Laminated onto the color layer is a second polyester supporting layer with an adhesive layer. This laminate takes the form of a multilayer composite and is applied to vehicle parts using techniques associated with pressure-sensitive self-adhesives, followed by removal of the first supporting layer abutting the clear layer, so that the clear layer forms a weather-resistant outer layer of the laminate. The PVC-containing color layer is flexible at room temperature and permits dimensional change within the laminate, which can therefore be laminated onto vehicle parts of three-dimensional shape. For durable and firm adhesive anchoring of the color layers, this laminate requires intermediate layers made from specific adhesives which have to fulfill certain preconditions.  
           [0003]    EP 0535504 B1 discloses a process for image transfer to coated surfaces, in particular those of timber-based materials, the surface being coated with a polymeric layer made from low-molecular-weight polymers and requiring curing by irradiation with electrons. The polymeric layer is brought into contact with a transfer medium bearing color pigment, with exposure to heat. There is diffusion of the color pigments into the polymeric layer. The irradiation with electrons cures the polymeric layer, crosslinking being undertaken with a radiation dose of from 40 to 80 kGray.  
         SUMMARY OF THE INVENTION  
         [0004]    It is an object of the invention to provide a method which produces an at least two-coloured multilayer composite, can be carried out cost-effectively on an industrial scale and which moreover manufactures the multilayer composite without adhesive layers and produces a multilayer composite whose decorative properties are durably resistant to the effects of weathering.  
           [0005]    According to the invention, the manner of achieving this object comprises, in a first step, partially curing the radiation-curable layers applied to supporting layers, and in a second step, completely curing the radiation-curable layers.  
           [0006]    The features of the method of the invention are that in the first step, the first radiation-curable layer, equipped with color pigments, is applied to a first supporting layer, that the second radiation-curable layer, equipped with color pigments, is applied to a second supporting layer, where the color pigments of the first layer differ from those of the second layer, that the two supporting layers are laminated, with the radiation-curable layers facing toward one another, to give a multilayer composite, and the radiation-curable layers are partially cured, and that in the second step, the multilayer composite is laminated with a plastic film to which a radiation-curable clear outer layer is applied, which faces toward the multilayer composite, and that the mutually abutting layers are completely cured.  
           [0007]    In executing the method, the partial curing and the complete curing of the layers is undertaken with the aid of actinic radiation. The actinic radiation used here comprises accelerated electrons, UV radiation in the wavelength region from 50 to 480 nm or X-ray radiation in the wavelength region from 0.05 to 8.0 nm.  
           [0008]    In another embodiment of the method, use is made of radiation-curable layers based on C1l-C6-alkyl acrylates and/or methacrylates, in particular those based on methyl acrylates or on ethyl acrylates and/or methacrylates.  
           [0009]    In one embodiment of the method, the dose of actinic radiation in the steps is adjusted so that the amount of radiation required for the complete curing of the radiation-curable layers is not applied until the final irradiation stage.  
           [0010]    If the amount of radiation theoretically needed for complete curing has been reached prior to the final irradiation stage, the bond strength of the clear outer layer can be adversely affected.  
           [0011]    Further embodiments of the method arise from the measures described in claims  7  to  15 .  
           [0012]    The invention uses the method to produce a two- or multicolored multilayer composite which is composed of a supporting layer with a smooth regular surface, which is a plastic film or is a phenolic-resin-impregnated paper web, and of two or three acrylic-based layers laminated to one another and radiation-cured and comprising different color pigments, and of an acrylic-based clear outer layer, and also of a peelable plastic film as protective layer.  
           [0013]    A multilayer composite of this type may be a decorative coating bonded to plates or panels made from layers of paper saturated with phenolic resins and/or with melamine resins, or made from cardboard packaging, from wood, from plastics, from resin-saturated compacted wood chips or the like, to give weather-resistant panels for outdoor use. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 shows a process diagram for the production of a single-coloured multilayer composite from a paper support, a curable color layer, a clear outer layer and a plastic film as in the prior art,  
         [0015]    [0015]FIG. 2 a  shows a diagram of the first step in the production of an at least two-coloured multilayer composite according to the invention,  
         [0016]    [0016]FIG. 2 b  shows a diagram of the second step in the production of an at least two-coloured multilayer composite,  
         [0017]    [0017]FIG. 2 c  shows a diagram of a second step modified from FIG. 2 b  for the production of a three-coloured multilayer composite according to the invention,  
         [0018]    [0018]FIG. 3 a  shows a diagram of a first step modified from FIG. 2 a  for the production of a metallic-effect multilayer composite according to the invention, and  
         [0019]    [0019]FIG. 3 b  shows a diagram of a second step modified from FIG. 2 b  for the production of a metallic-effect multilayer composite. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    In FIG. 1, a phenolic-resin-saturated or phenolic-resin-impregnated paper substrate  31  has been wound up on a feed roll  20 . The web of paper substrate  31  is unwound from the feed roll  20  and passes through a printing unit  21  where screen printing or stencil printing is used to apply a color layer  151 . This color layer  151  is an acrylic layer which comprises color pigments.  
         [0021]    Furthermore, a plastics film  71  which has been wound onto a plastics film feed roll  51  is unwound and passed through another printing unit  61 . In this printing unit  61 , screen printing or stencil printing is used to apply a colorless protective layer to the plastic film  71 , examples of which are a polyolefin, such as polyethylene or polypropylene, a polyester or the like. The colorless protective layer  171  is also acrylic-based. The paper substrate  31  with the color layer  151  and the plastics film  71  with the colorless protective layer  171  are brought together in a laminating unit  41  and laminated to each other with the aid of heat and/or pressure, to give a multilayer composite  91 . After leaving the laminating unit  41 , the multilayer composite  91  passes through a curing unit  81 , in which accelerated electrons are used for complete curing of the two mutually abutting layers, namely the color layer  151  and the protective layer  171 , thus forming a solid composite. The multilayer composite  91  is a single-coloured laminate with an optically clear outer layer or protective layer  171 , and is wound up onto a multilayer composite feed roll  101 .  
         [0022]    This known process does not permit production of a two- or multicolored multilayer composite comprising curable layers without the use of adhesive layers, since complete curing of the layers takes place immediately after lamination of the layers to give the multilayer composite. This means that bonding between further layers is not possible without resorting to the use of adhesive layers.  
         [0023]    The curing of the layers may be undertaken with actinic radiation quite generally, but specifically for the purposes of this invention the radiation is preferably highly accelerated electrons, as preferably used in the curing unit  81 , and UV radiation in the wavelength region from 50 to 480 nm or X-ray radiation in the wavelength region from 0.05 to 8.0 nm.  
         [0024]    For the curing by irradiation with accelerated electrons, a maximum dose of up to 60 kGray is generally sufficient. After curing with a dose of this type, the majority of the reactive acrylic groups have reacted, and the cured layers are completely dry and solid. If the curable layers are irradiated with a dose lower than that given above the curing achieved is only partial, meaning that there are still sufficient residual reactive acrylic groups which can react with another acrylic layer. This is the inventive concept on which the method of the invention is based, as is described in more detail with reference to FIGS. 2 a  to  3   b.    
         [0025]    In a first step, as shown in FIG. 2 a,  a first supporting layer  3 , for example a film web or a phenolic-resin-saturated paper web, is equipped with a first radiation-curable layer  15 . This first curable layer is an acrylic-based layer and comprises color pigments of a particular color. The supporting layer  3  has been wound up on a first feed roll  1  and, after unwinding, is passed through a first printing unit  2 , in which screen printing or stencil printing is used to apply the first curable layer  15 . A second supporting layer  7 , for example a plastics film, has been wound up as a web on a second feed roll  5 . This second supporting layer  7  is unwound from the second feed roll  5  and passes through a second printing unit  6 , in which screen printing or stencil printing is used to apply a second radiation-curable layer  16  which comprises color pigments of a particular color. These color pigments differ from the color pigments in the first curable layer  15 . The two supporting layers  3  and  7  are brought together with the layers  15 ,  16  facing toward one another immediately prior to a first laminating unit  4 , in which they are pressed together with exposure to heat and/or pressure to give a multilayer composite  9 , a section of which is shown in detail at A. After leaving the laminating unit  4 , the multilayer composite  9  passes through a first curing unit  8 , in which accelerated electrons partially cure the two layers  15 ,  16 . The electron radiation dose is in the range from 0.5 to 30 kGray and is insufficient for complete curing of the two layers. The curing may also be carried out using UV radiation in the wavelength region from 50 to 480 nm or X-ray radiation in the wavelength region from 0.05 to 8.0 nm, instead of accelerated electrons. The partial curing of the layers  15 ,  16  takes place in the first step using not more than 30% of the maximum radiation dose required for complete curing of the layers. This gives a two-coloured multilayer composite  9 , which is wound up onto a multilayer composite feed roll  10 . There then follows the second step, as shown in FIG. 2 b,  in which the two-coloured multilayer composite  9  passes through the existing system for a second time. To this end, the multilayer composite feed roll  10  replaces the first feed roll  1  in the first step, and the multilayer composite  9  is unwound from the feed roll  10 . Prior to the second step it is useful for the second supporting layer  7  to be removed from the multilayer composite  9 . A section of the multilayer composite  9 , without the supporting layer  7  which has been removed, is shown in detail at B. The composite  9  passes through the first printing unit  2  which has been taken out of operation in the present instance, since there is no further color layer to be applied.  
         [0026]    A web-shaped plastics film  12  is unwound from a third feed roll  11 , and passes through the second printing unit  6 , in which screen printing or stencil printing is used to apply an optically clear layer, namely what is known as the clear outer layer  17 . Although no color pigments are present in this layer, it is similar to the layers  15  and  16  in being an acrylic layer curable by actinic radiation. The multilayer composite  9  and the plastics film  12  which acts as a protective film, with the clear outer layer  17  which has been applied, are brought together prior to or in the laminating unit  4 , and laminated to each other in this laminating unit by means of heat and/or pressure. After leaving the laminating unit  4 , the laminate made from two-colored multilayer composite  9  and from the clear outer layer  17  together with the plastics film  12  passes through the curing unit  8 , which in the second step uses the accelerated electrons at full power, i.e. a dose of from 1.65 to 100 kGray, to bond the clear outer layer  17  with the two layers  15  and  16  which have previously been partially cured. The full radiative power of the curing unit  8  completely cures the curable layers  15 ,  16 ,  17 , and these form a dry and firmly bonded ply within the two-colored multilayer composite  13 . This multilayer composite  13  is wound up onto a multilayer composite feed roll  14 . A section of the multilayer composite  13  is shown in detail and at C on an enlarged scale.  
         [0027]    It is also possible to apply a still further color layer in the second step, thus obtaining a three-colored multilayer composite. The only requirement for this is that, in what is known as the modified second step shown in FIG. 2 c,  during passage of the two-colored multilayer composite  9  through the first printing unit  2  screen printing or stencil printing is used to apply a radiation-curable layer  18  which has been equipped with color pigments. A section of the multilayer composite with the curable layers  15 ,  16  and  18 , and also with the first supporting layer  3 , is shown in detail at D. In other respects the procedure is unchanged from the second step as shown in FIG. 2 b.  A section of the resultant multilayer composite  19  made from a first supporting layer  3 , the curable layers  15 ,  16  and  18  with color pigments, the clear outer layer  17  and the protective film  12  is shown in detail at E.  
         [0028]    Examples of the curable layers  15 ,  16 ,  17  and  18  used are those based on C1-C6-alkyl acrylates and/or methacrylates, in particular based on methyl acrylates or on ethyl acrylates and/or methacrylates. Alongside these, use may also be made of comonomer units, in particular acrylonitrile or alkyl vinyl ethers.  
         [0029]    A very general rule for the method is that the dose of actinic radiation which is applied in the curing unit  8  has been adjusted so that the amount of radiation required for complete curing of the curable layers is not applied until the final irradiation stage. In the first step, therefore, the curable layers  15 ,  16  are brought into contact with not more than 30% of the maximum dose of actinic radiation required for full curing. In the second step, the two partially cured layers  15 ,  16  and the clear outer layer  17  are brought into contact with a dose of from 30% to 100% of the actinic radiation for full curing. This again applies to the case where another curable layer  18  with color pigments is additionally applied to the multilayer composite  9  in the modified second step. The color of these color pigments differs from that of the color pigments of the curable layers  15  and  16 . It is preferable for the color pigments of the layers  15 ,  16  and  18  to be selected from the group consisting of metal oxides, metal hydroxides and metal oxide hydrates, sulfur-containing silicates, metal sulfides, metal selenides, complex metal cyanides, metal sulfates, metal chromates, metal molybdates, azo pigments, indigoids, dioxazine pigments, quinacridone pigments, phthalocyanine pigments, isoindolinone pigments, perylene pigments, perinone pigments, metal complex pigments, alkali blue pigments and diketopyrrolopyrrole (DPP) pigments.  
         [0030]    In another embodiment of the method, the curable layers  15  to  18  may be applied to the associated supporting layers by casting or by printing rollers, instead of by screen printing or stencil printing.  
         [0031]    The plastics of the web-shaped plastics films are particularly selected from the group consisting of polyolefins, such as polyethylene and polypropylene, and polyesters, or from the group consisting of polyamides.  
         [0032]    The two- or multicolored multilayer composite  13  obtained by the method is therefore composed of a supporting layer, which is a film or a phenolic-resin-impregnated paper web  3 , of two or three acrylic-based, cured layers  15 ,  16 ,  18  laminated to one another and each comprising a different color pigment, of an acrylic-based clear outer layer  17 , and also of a peelable plastics film  12  as protective layer. This multilayer composite  13  is preferably used as a decorative coating bonded to sheets made from layers of paper saturated with phenolic resins and/or with melamine resins, or made from cardboard packaging, from wood, from plastic, from resin-saturated compacted wood chips or the like. Applying this multilayer composite  13  to sheets of this type gives weather-resistant panels or decorative plates for outdoor use on buildings, for example as cladding, or in indoor areas subject to moisture.  
         [0033]    In the modified first step as described in FIG. 3 a,  a third web of supporting layer  24  has been wound up on a fourth feed roll  22 . An example of the supporting layer  24  is a plastics film, preferably a polypropylene film with a very uniform, smooth surface. The supporting layer  24  is drawn off from the feed roll  22 , and a metallic coating  23  is applied to the supporting layer  24 . The metallic coating  23  is composed of an acrylate-based layer in which color pigments have been dispersed. The color pigments are preferably metal oxide pigments, in particular aluminum oxide pigments which give the metallic coating  23  a metallic color.  
         [0034]    A fourth supporting layer  25  has been wound up on a sixth feed roll  36 , and after this layer has been unwound from the feed roll  36  it receives an application of a radiation-curable clear layer  26 . The supporting layer  25  is again a plastics film with a smooth uniform surface. With the metallic coating  23  and clear layer  26  facing toward one another, the two supporting layers  24 ,  25  are brought together in a second laminating unit  27 , where they are laminated by means of pressure and/or heat, to give a multilayer composite  28 , which immediately after emerging from the laminating unit  27  passes through a second curing unit  29 . In this curing unit  29 , the clear layer  26  and the metallic coating  23  are partially cured by an electron beam with a dose of from 2 to 30 kGray, followed by winding-up onto a multilayer composite feed roll  30 . A section of the multilayer composite  28  is shown in detail at F.  
         [0035]    In the modified second step shown in FIG. 3 b,  the multilayer composite feed roll  30  is on the right-hand side, and when the multilayer composite  28  is unwound from the feed roll  30  the supporting layer  24  is simultaneously removed from the metallic coating  23 . A section of the multilayer composite  28 , without the supporting layer  24 , is shown in detail at G.  
         [0036]    On the left-hand side of FIG. 3 b  a fifth supporting layer  33  wound up on a fifth feed roll  32  has a highly non-uniform surface, on which, after unwinding from the feed roll  32 , a primer layer and/or adhesion-promoter layer  34  made from a radiation-curable primer and/or from a radiation-curable adhesion promoter is applied. A section through the supporting layer  33  and the primer layer and/or adhesion-promoter layer  34  is shown in detail at H. An example of the supporting layer  33  is a phenolic-resin-impregnated paper web, or what is known as a kraft paper.  
         [0037]    The multilayer composite  28 , without the supporting layer  24 , and the supporting layer  33  with the primer layer and/or adhesion-promoter layer  34  applied are brought together in the second laminating unit  27 , where they are pressed together using heat and/or pressure, to give a multilayer composite  35 . The multilayer composite  35  then passes through the second curing unit  29 , in which accelerated electrons are used for complete curing of the layers  23 ,  26  and  34  in the multilayer composite  35 , using a dose of from 6.7 to 100 kGray from the electron beam. A section of the structure of the multilayer composite  35  is shown in detail at I. The multilayer composite  35  is wound up onto a multilayer composite feed roll  37  and used for further processing as a decorative coating for plates or panels.  
         [0038]    The curing unit  29  may also be a UV or X-ray unit, in which case the curing of the layers takes place with the aid of UV radiation in the wavelength region from 50 to 480 nm or X-ray radiation in the wavelength region from 0.05 to 8 nm.