Patent Publication Number: US-2023147373-A1

Title: Image transfer sheet, manufacturing method for image transfer sheet, and image transfer method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. patent application Ser. No. 16/610,744 filed Nov. 4, 2019, which is a Section 371 of International Application No. PCT/JP2018/029493, filed Aug. 6, 2018, which was published in the Japanese language on Jan. 30, 2020 under International Publication No. WO 2020/021727 A1, which claims the benefit of Japanese Patent Application No. 2018-139897 filed Jul. 26, 2018. The present application claims the benefit of each of the above-identified applications. The disclosures of each of the above-mentioned applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an image transfer sheet for transferring any image onto a transfer target object; and a method for manufacturing the image transfer sheet or a method for transferring an image onto a transfer target object using the image transfer sheet. 
     BACKGROUND OF THE INVENTION 
     Technologies for transferring an image onto a surface of an item, on which it is difficult to print an image directly, include a transfer technique with a substrate, wherein an image layer is printed on a transparent thin film-like substrate provided with an adhesive layer on its back surface, and wherein the entire substrate is affixed onto an item surface; and a transfer technique without a substrate, wherein eventually only an image layer is transferred directly onto an item surface. The former technique is generally known by “image stickers,” “cutting sheets,” etc., wherein the transferred image is spaced from the item surface by the interposing substrate thickness, thereby generating an uneven image surface as a result. Also, work is required to remove unwanted substrate parts bearing no image before or after the image transfer using a tool such as a cutting plotter. On the other hand, the transfer technique without a substrate enables a high-quality image transfer as if the transferred image was directly printed on an item surface, and this technique does not require the work to remove the non-image part of the substrate. 
     Known transfer techniques without a substrate include the water transfer, the thermal transfer, the sublimation transfer, the dry transfer, etc. Among others, the dry transfer is performed by affixing and pressing an image transfer sheet onto a surface of a transfer target object, wherein the image transfer sheet is defined by providing an image layer on the lower surface of a transfer film and applying adhesive thereover, and transferring only the image layer onto the transfer target object surface by adhesion, wherein no water or heat is used to allow high-quality image transfer. 
     The present inventors conducted research of the transfer technique without a substrate, and developed and received a patent right for an image transfer sheet, a method for manufacturing the image transfer sheet and a method for transferring an image using the method for manufacturing the image transfer sheet described in Patent Document 1. The image transfer sheet according to the present patent invention is defined by providing an image layer in part of an area between a transfer film as an upper layer and a glue layer as a lower layer, and releasably gluing release paper onto the lower glue layer, wherein the transfer film is configured to be releasable from the image layer, but easily adhesive to the glue layer. During the transfer, the image transfer sheet with the release paper peeled off is pressure-glued onto a transfer target object, and then, the transfer film is peeled off. At this time, the image layer is glued and transferred onto the transfer target object by the glue layer, but the glue layer part without the image layer is peeled and removed from the surface of the transfer target object together with the transfer film. 
     Patent Document 1: JP-A-2017-209931 
     The image transfer sheet according to the invention is based on a reversed concept to print an image layer of a normal image directly on a surface of a glue layer of a glued sheet prepared in advance, and then, cover the glued sheet with a transfer film to complete an image transfer sheet. Such an improvement solved problems such as the difficulty in even application of the glue layer during the manufacturing stage of conventional dry image transfer sheets and/or the laborious removal of the glue left on the part bearing no image layer after the image transfer. Also, the image transfer sheet according to the invention allowed transferring of an image layer with a three-dimensional feel by forming the image layer using the UV printing. Since the normal image is directly printed on the glue layer surface, the raised printed surface specific to the UV printing was enabled and the problematic air bubbles in and wrinkles of the image layer, often seen in the UV printing, have been successfully prevented. 
     However, the image transfer sheet according to the invention still had an unresolved issue of air left between the transfer target object and the glue layer, generating air bubbles during the image transfer onto an item. Since the image transfer sheet is ultimately affixed onto the transfer target object by hand, it is difficult to completely prevent the air bubbles from forming. Air bubbles not only compromise the aesthetic and quality of the transferred image, but also make the image layer susceptible to peel-off when the air bubbles form on the outer edge of the transferred image. Also, the air bubbles tend to form more as the size of transferred image increases and when the surface of the transfer target object is curved. 
     SUMMARY OF THE INVENTION 
     In order to address the above and other problems of the conventional art, the purpose of the present invention is to provide an image transfer sheet for leaving no air bubbles between an image layer and a transfer target object, a method for manufacturing the image transfer sheet and a method for transferring an image. 
     In order to overcome the above challenges, an image transfer sheet of Claim  1  of the present invention comprises an image layer in part of an area between a transfer film as an upper layer and a glue layer as a lower layer, and the image transfer sheet is defined by the glue layer, a lower surface of which being provided with many individually independent fine protrusions in the entire lower surface, and a release paper releasably glued to the lower surface of the glue layer, wherein the transfer film is releasable from the image layer and easily adhesive to the glue layer, and wherein a lower surface of the transfer film is in contact with the image layer where there is the image layer, and is in contact with the glue layer where there is no image layer. 
     The transfer film as the upper layer is preferably transparent PET (polyethylene terephthalate) film with a surface processed to be releasable, but is not limited by it and may be any film whose lower surface is releasable from the image layer and easily adhesive to the glue layer. For example, if the glue is common pressure-sensitive adhesive containing acrylic resin as its main component, the transfer film is preferably film with the same acrylic resin. 
     Also, the shape of the many fine protrusions on the lower surface of the glue layer may be either dot-like or line-like, but in either way, it is desirable that the height of the protrusions is 5-20 μm, that when the protrusions are dot-like, the diameter and the interval between adjacent dot-like protrusions are 0.2 mm-1.0 mm, and that when the protrusions are line-like, the line width and the interval between adjacent line-like protrusions are also at least 0.2-1.0 mm. 
     According to such a structure, the lower surface of the glue layer having the protrusions is exposed by peeling and removing the releasable release paper from the image transfer sheet. By pressure-gluing the glue layer onto the surface of the transfer target object and applying pressure on the transfer film surface, the image transfer sheet is affixed onto the transfer target object by the glue layer adhesion. At this time, by applying pressure selectively onto the part bearing the image layer, most of the air remaining between the glue layer and the transfer target object where there is the image layer moves through between the protrusions toward the part where there is no image layer, or released to outside from the edge of the image transfer sheet, wherein a trace amount of remaining air is also dispersed among the gap space between the protrusions, leaving no air bubbles recognizable by the naked eye. 
     Subsequently, by peeling and removing the transfer film of the top surface of the image transfer sheet, the image layer remains on the glue layer where there is the image layer, from which the transfer film is releasable. On the other hand, where there is no image layer, the glue layer remains glued to the lower surface of the transfer file, and peeled and removed from the surface of the transfer target object. As a result, only the image layer remains glued onto the surface of the transfer target object by the glue layer, to thereby complete the image transfer. 
     Next, the invention of Claim  2  is the image transfer sheet of Claim  1 , wherein the image layer is ink printed on an upper surface of the glue layer by an inkjet printer. Also, the invention of Claim  3  is the image transfer sheet of Claim  2 , wherein the image layer is UV ink printed on the upper surface of the glue layer by a UV printer. 
     Non-contact printers are inkjet printers, UV printers, and the like, and are a type of printers which drop ink directly onto the printing surface without any interposing plate material or transfer body directly in contact with the printing surface. The image layer according to the present invention is glued to the transfer target object via the glue layer as the ink particles are dropped and deposited on the upper surface of the glue layer, and dried and cured to form a film. The UV ink is formulated with a photoinitiator which triggers a photopolymerization reaction by UV light, monomers/oligomers which polymerize by the photopolymerization reaction and pigments as dyes. 
     Next, the invention of Claim  4  is a method for manufacturing an image transfer sheet, the method comprising the steps of: forming a glue layer by pressure-gluing a glued sheet, defined by a substrate and a glue layer provided on a lower surface of the substrate, onto an upper surface of a release paper, having many fine recesses, and subsequently peeling and removing the substrate and exposing the glue layer; printing ink on the exposed upper surface of the glue layer by a non-contact printer to thereby provide an image layer on part of the glue layer; and affixing a transfer film on the glue layer provided with the image layer, wherein the transfer film is releasable from the image layer and easily adhesive to the glue layer, and causing the transfer film to contact with a lower surface of the transfer film onto the image layer where there is the image layer, and with the glue layer where there is no image layer. Also, the invention of Claim  5  is the method for manufacturing the image transfer sheet of Claim  4 , wherein the image layer is UV ink printed on the upper surface of the glue layer by a UV printer. 
     Next, the invention of Claim  6  is an image transfer method, comprising the steps of: forming a glue layer by pressure-gluing a glued sheet, defined by a substrate and a glue layer provided on a lower surface of the substrate, onto an upper surface of a release paper, having many fine recesses, and subsequently peeling and removing the substrate and exposing the glue layer; printing ink on the exposed upper surface of the glue layer by a non-contact printer to thereby provide an image layer on part of the glue layer; affixing a transfer film on the glue layer provided with the image layer, wherein the transfer film is releasable from the image layer and easily adhesive to the glue layer, and causing the transfer film to contact with a lower surface of the transfer film onto the image layer where there is the image layer, and with the glue layer where there is no image layer; peeling and removing only the release paper from the image transfer sheet; pressure-gluing the lower surface of the glue layer of the image transfer sheet, with the release paper removed, onto a transfer target object; and peeling and removing the transfer film from the image transfer sheet pressure-glued to the transfer target object. Also, the invention of Claim  7  is the image transfer method of Claim  6 , wherein the image layer is UV ink printed on the upper surface of the glue layer by a UV printer. 
     In the conventional image transfer sheets, there were possibilities for air bubbles to remain between the post-transfer image layer and the transfer target object, but in accordance with the image transfer sheets, the methods for manufacturing the image transfer sheets and the methods for transferring an image according to the present invention, at least air bubbles recognizable by the naked eye may be prevented. Thus, the present invention provides an effect in which image transfer work susceptible to air bubbles with the conventional art, such as transferring a relatively large image, image transfer onto a transfer target object with a curved surface, etc. may be performed easily and reliably. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view showing one example of an image transfer sheet  1  according to a first embodiment; 
         FIG.  2    is a cross-sectional view showing one example of an image transfer sheet  2  according to a second embodiment; 
         FIG.  3    is a cross-sectional view showing the glue layer forming step (pressure-gluing of release paper) of the first embodiment; 
         FIG.  4    is a cross-sectional view showing the glue layer forming step (forming of fine protrusions) of the first embodiment; 
         FIG.  5    is a cross-sectional view showing the glue layer forming step (peeling removal of a substrate) of the first embodiment; 
         FIG.  6    is a cross-sectional view showing the printing step of the first embodiment; 
         FIG.  7    is a cross-sectional view showing the affixing step of the first embodiment; 
         FIG.  8    is a cross-sectional view showing the release paper removing step of the first embodiment; 
         FIG.  9    is a cross-sectional view showing the pressure-gluing step of the first embodiment; 
         FIG.  10    is a cross-sectional view showing the file peeling step of the first embodiment; 
         FIG.  11    is a cross-sectional view showing the glue layer forming step (glued sheet) of the second embodiment; 
         FIG.  12    is a cross-sectional view showing the glue layer forming step (glued roller) of the second embodiment; 
         FIG.  13    is a cross-sectional view showing the glue layer forming step (forming protrusions on the glue layer) of the second embodiment; 
         FIG.  14    is a cross-sectional view showing the glue layer forming step (affixing release paper  1 ) of the second embodiment; 
         FIG.  15    is a cross-sectional view showing the glue layer forming step (affixing the release paper  2 ) of the second embodiment; 
         FIG.  16    is a cross-sectional view showing the glue layer forming step (peeling removal of the substrate) of the second embodiment; 
         FIG.  17    is a cross-sectional view showing the printing step of the second embodiment; and 
         FIG.  18    is a cross-sectional view showing the affixing step of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the technology will now be described with reference to the drawings.  FIG.  1    is a cross-sectional view showing an example of an image transfer sheet  1  according to a first embodiment of the present invention; and  FIG.  2    is a cross-sectional view showing an example of an image transfer sheet  2  according to a second embodiment of the present invention. The image transfer sheets  1 ,  2  have different cross sectional shapes for release paper  13  depending on the different manufacturing methods, but otherwise have a common structure. 
     The image transfer sheet  1  comprises an image layer  15  between a transfer film  16  as an upper layer and a glue layer  12  as a lower layer, and the glue layer  12  is defined by having its lower surface releasably glued to an upper surface of the release paper  13 . The release paper  13  according to the first embodiment is provided with many individually independent fine recesses  13   a  across its upper surface, and the lower surface of the glue layer  12  has many individually independent fine protrusions  12   a  formed thereon so that the protrusions  12   a  mate with the recesses  13   a.  Note that the image layer  15  is printed directly on the upper surface of the glue layer  12  by a non-contact printer such as an inkjet printer, an UV printer or the like, wherein in the case of an inkjet printer, the image layer  15  is an ink layer, and wherein in the case of an UV printer, the image layer  15  is an UV ink layer which has been dried and cured into a thin film-like form through the photopolymerization reaction by UV radiation during the printing. 
     The material of the transfer film  16  is preferably transparent acrylic resin if the glue component of the glue layer  12  is acrylic adhesive as described below, but not necessarily limited to acrylic resin and may be any material releasable from the image layer  15  and easily adhesive to the glue layer  12 . Also with a transparent transfer film  16 , the image shape and colors may be visually recognized through the surface of the image transfer sheet  1 , and therefore, it will be easier to perform positioning and color matching of the transparent transfer film  16  against a transfer target object O. 
     The glue layer  12  is, for example, a layer of pressure-sensitive adhesive formulated by acrylic resin and tackifier, and may have any thickness, but preferably of 10-100 μm, in particular, about 30 μm. The release paper  13  has a silicon film or the like at least on its upper surface, and the release paper  13  is releasable from the glue layer  12  and easily peels off at a normal temperature. 
     (A Method for Manufacturing the Image Transfer Sheet  1  and a Method for Transferring an Image According to the First Embodiment) 
       FIG.  3   - FIG.  7    are simplified cross-sectional views showing a method for manufacturing the image transfer sheet  1  according to the first embodiment of the present invention step by step. First,  FIG.  3   - FIG.  5    show the glue layer forming steps, wherein a glued sheet  10 , defined by the glue layer  12  provided on the lower surface of a substrate  11 , is pressure-glued to the release paper  13 , provided with the many fine recesses  13   a  across the upper surface of the release paper  13  ( FIG.  3   ) to cause the lower surface of the glue layer  12  to enter into the recesses  13   a  to form the many independent fine protrusions  12   a  ( FIG.  4   ), and subsequently the substrate  11  is peeled and removed to expose the upper surface of the glue layer  12  ( FIG.  5   ). For the glued sheet  10 , pressure-sensitive adhesive consisting of acrylic resin, polyolefin fiber, tackifier may be used as the glue layer  12 ; and ready-made pressure-sensitive double-sided adhesive tape using siliconized glassine paper may be used as the substrate  11 . Note that the release paper  13  with the many fine recesses  13   a  in the upper surface thereof may be made by embossing the release paper  13  itself to provide the recesses  13   a  as illustrated, or, when siliconizing and conferring the releasability to the substrate surface of the release paper during its manufacturing, the silicon film may be embossed before it solidifies to provide the recesses  13   a.    
       FIG.  6    shows the printing steps, wherein an image layer  15  is printed directly on the upper surface of the glue layer  12  by a non-contact printer. Specifically, the release paper  13  provided with the glue layer  12  is set in the non-contact printer, and then, ink of an arbitrary color is sprayed and printed directly onto the upper surface of the glue layer  12  from a printer head P. In the case of UV printing, by radiating UV light from a UV light source (not shown) immediately after UV ink is sprayed, ink particles are dried and cured through the photopolymerization reaction to thereby form a film-like image layer  15 . Since the UV ink particles adhering to the glue layer  12  are dried and cured after they form sealing contact with the upper surface&#39;s recesses/protrusions of the glue layer  12 , the UV ink particles are fixedly attached to the glue layer  12 . Also, since the image layer  13  is printed as a normal image, performing the thick printing as a UV printing characteristic allows any printing thickness with a three-dimensional feel within specifications of the printer in use, and applying the thick printing for only a specific part of the printed image enables creating an accent to the image&#39;s three-dimensional feel. 
       FIG.  7    shows the affixing step, wherein, after the image layer  15  has been dried and fixed, the transfer film  16  is affixed onto the glue layer  12 , which includes the image layer  15 , with the transfer film  16  covering the range of the entire image, to thereby complete the image transfer sheet  1  shown in  FIG.  1   . The completed image transfer sheet  1  may be handled in a similar manner to that of the conventional stickers, and may be regarded as a product allowing transfer and printing of only images. 
       FIGS.  8 - 9    are simplified cross-sectional views showing the steps of transferring the image layer  15  onto the surface of the transfer target object O.  FIG.  8    shows the step of peeling and removing the release paper, wherein the release paper  13  is peeled and removed from the image transfer sheet  1 . On the lower surface of the exposed glue layer  12 , the many independent fine protrusions  12   a  are formed corresponding to the recesses  13   a  in the upper surface of the release paper  13 . 
       FIG.  9    is a cross-sectional view showing the pressure-gluing steps of positioning the image transfer sheet  1  on the surface of a desired transfer target object O after the release paper  13  is peeled and removed, and pressure-gluing the lower surface of the glue layer  12  upon the transfer target object O. By applying pressure over the transfer film  16 , the lower surface of the glue layer  12  consisting of pressure-sensitive adhesive is glued onto the surface of the transfer target object O, but at this time, it is desired to apply pressure only over the range of the image layer  15 . This pressure may be applied through rubbing by finger tips, but in order to prevent wrinkling and/or deformation of the image layer  15 , it is desirable to use a roller, a squeegee (spatula) or the like having even, smooth and appropriate resiliency to apply an uniform pressure. 
     At this time, by applying the pressure only over the range of the image layer  15 , most of the air between the part of the glue layer  12  having the image layer  15  and the transfer target object O moves towards the part of the glue layer  12  without the image layer  15  through gaps between the protrusions  12   a,  or released to outside from the edge of the image transfer sheet  1 , wherein a trace amount of remaining air is also dispersed among the gap space between the protrusions, leaving no air bubbles recognizable by the naked eye. 
       FIG.  10    is a cross-sectional view showing the film peeling step, wherein the transfer film  16  is peeled off after pressure-gluing of the image transfer sheet  1 . When the transfer film  16  is peeled off after the image transfer sheet  1  is pressure-glued, the image layer  15 , being glued to the glue layer  12 , remains on the surface of the transfer target object O, to thereby complete the image transfer. On the other hand, since the glue layer  12  in the range without the image layer  15  is glued to the transfer film  16  and is removed from the surface of the transfer target object O, extra glue outside of the image may be prevented from remaining on the transfer target object O. Note that coating may be applied over the transfer target object O after the completion of the image transfer in order to improve abrasion resistance of the image layer  15  after its transfer and/or prevent peeling of the image layer  15  due to deterioration of the glue layer  12 . 
     (A Method for Manufacturing the Image Transfer Sheet  2  and a Method for Transferring an Image According to the Second Embodiment) 
       FIGS.  11 - 18    are cross-sectional views showing a method for manufacturing the image transfer sheet  2  up to the glue layer forming step, printing step and affixing step according to the second embodiment of the present invention. The method for manufacturing the image transfer sheet  2  according to the second embodiment is similar to its counterpart of the first embodiment, except that the second embodiment has different means for forming the glue layer  12  with the many independent fine protrusions  12   a  provided on the lower surface of the glue layer  12  compared to the first embodiment; from the release paper removing step and thereafter, the second embodiment is similar to the first embodiment. 
       FIGS.  11 - 16    show the glue layer forming steps. First, a glued sheet  10 , which has a glue layer  12  provided on the lower surface of the same substrate  11  used in the step of manufacturing the image transfer sheet  1  according to the first embodiment, is flipped to have its glue side facing upward ( FIG.  11   ). Next, a roller R having many independent fine protrusions r on its peripheral surface is prepared, and after applying a paste material p, with the same material as that of the glue layer  12 , only on the surface of the protrusions r, the roller R is pressed against and rolled on the glue layer  12  of the glued sheet  10  ( FIG.  12   ). Thus, the paste material p sequentially leaves the protrusions r to adhere on the surface of the glue layer  12  to form the many independent fine protrusions  12   a  ( FIG.  13   ). Next, on the surface of the glue layer  12 , where the many independent fine protrusions  12   a  are formed, release paper  14 , surface of which is not embossed and smooth, is pressure-glued ( FIG.  14   ,  FIG.  15   ), and then, the substrate  11  is peeled off after the entirety is reversed upside down to expose the smooth surface of the glue layer  12  ( FIG.  16   ). Note that the shape of the many independent fine protrusions r on the peripheral surface of the roller R may be either dot-like or line-like, wherein when the protrusions r are dot-like, the protrusions  12   a  formed on the glue layer  12  will be dot-like, and wherein when the protrusions r are in a line-like shape, the protrusions  12   a  will be line-like. 
       FIG.  17    shows the printing step and  FIG.  18    the affixing step, wherein operations of these steps are basically similar to those of the first embodiment, and with these steps, the image transfer sheet  2  according to the second embodiment, as shown in  FIG.  2   , is completed. Also, the release paper removal step of removing only the release paper  14  from the image transfer sheet  2 , the pressure-gluing step of pressure-gluing the glue layer  12  of the image transfer sheet  2  after removing the release paper  14 , and the film peeling step of peeling and removing the transfer film  16  from the image transfer sheet  2  pressure-glued to the transfer target object O are also similar to their counterparts in the first embodiment. 
     Although specific structures of the image transfer sheets, the methods for manufacturing the image transfer sheets and the methods for transferring an image according to the present invention have been described, the present invention is not limited to the above-described embodiments and allows improvements or modifications within the scope of technical ideas of the present invention; therefore, those improvements or modifications fall within the technical scope of the present invention. For example, the image layer  15  may be provided as in the image transfer sheet according to the second embodiment described in Patent Document 1, where the image layer is provided in a double-layer structure, in which case even when the image layer is transferred onto a highly-transparent transfer target object such as glass, each image layer seen from both sides of the transfer target object may be clearly recognizable without the image layer on the opposite side visible through the transparent transfer target object; or the image layer  15  may be provided as in the image transfer sheet according to the third embodiment of Patent Document 1, where a masking layer is further provided between image layers of the double-layer structure to enable changing of the color development and/or tone of each image layer. 
     Also, the present invention has a structure in which the transfer film is releasable from the image layer and easily adhesive to the glue layer, but the releasability between the transfer film and the image layer changes depending on the density of the ink constituting the image layer. Thus, adjusting the density of the ink used to print the image layer allows to control whether the image layer is transferred or peeled off together with the glue layer when peeling and removing the transfer film. According to the results of trials conducted by the present inventors using a UV printer (UFJ6042Mk2, Mimaki Engineering Co., Ltd.) and black ink (LUS-150 ink), when a light-colored image was printed with the ink density per unit area under 2 nanoliter/square mm, it was confirmed that upon peeling and removing of the transfer film after the image transfer, the image layer was peeled off from the transfer target object without remaining transferred. Note that the threshold of the ink density per unit area may be determined according to the ink and printer specifications. With such an effect as above, when printing the image layer, part of an image to be transferred may be printed with desired colors of ink each having a density equal to or higher than the threshold, whereas an image with no need to be transferred may be printed with a density less than the threshold to thereby allow having only desirable images on the image transfer sheet transferred and unwanted images not transferred. For example, on the image transfer sheet, crop marks for positioning may be printed as light-colored images so that the crop marks are not transferred after the transfer to thereby improve accuracy and efficiency of the transfer work. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, since air bubbles may be prevented from remaining between the transfer target object and the glue layer in the prior-art dry image transfer sheets with no substrate, air bubbles compromising aesthetics and/or quality of the transferred image may be prevented, and durability of the transferred image may be also improved. Thus, compared to the conventional art, transfer of a large image and/or image transfer to an item with a curved surface will be easier and the applications may be broadened.