DESCRIPTION

FIELD OF THE INVENTION

Printed PVC films are widely used to decorate flooring, furniture panels and profiles. Usually, PVC films are printed by means of gravure printing using solvent-based inks. Recently, in order to meet environmental criteria, the industry is trying to use water-based inks which, however, have no affinity with thermoplastic film and offer inadequate print quality.

The evaporation of water also requires the use of heat which makes its application on PVC film critical, which is known to be thermosensitive, causing elongation and distortion .

In particular, the reproduction of wood on various materials represents a typical application of digital printing for the production of floors, furniture panels, skirting boards, profiles and in general in the field of design and architecture.

In many applications, the decorated PVC film is protected by laminating a transparent PVC film on it. The lamination layer is normally applied by means of pressure and heat, with calenders or presses, without the use of adhesives.

Digital printing and in particular inkjet printing is increasingly establishing itself in industrial sectors, replacing traditional methods based on analogue printing. The advantages of digital printing are considerable and include high flexibility, the ability to produce short runs and the reduction of consumable waste.

In the recent years, PVC flooring is experiencing a renaissance with the introduction of the so-called LVT (Luxury Vinyle Tile) where the product is presented as a plank suitable to be placed floating or self-adhesive and characterized by low thickness useful in renovations. Since LVT is a design product, the market would welcome the use of digital technologies for printing, allowing just-in-time production and the proposal of fast creations, typical of the fashion industry.

To ensure the adhesion of the transparent protective layer (commonly called wear layer), the ink should have thermoplastic behavior. In fact, the rotogravure inks currently used are mainly solvent based and generally contain solubilized or dispersed PVC which is contributing to bonding between decorative and wear layer.

Radiation curable inks are widely used for inkjet printing as they are characterized by high in-machine stability, are cured instantly by controlling drop deposition and resulting image quality. Radiation curable inks do not have thermoplastic behavior. The addition of thermoplastic resins, while giving partial thermoplastic behavior to the system, negatively affects its printability. On the other hand, aqueous-based inks can be added with thermoplastic resins in dispersion or emulsion, have a low dry content which facilitates the adhesion of the lamination layer but have little affinity for plastic materials. In fact, water-based inks are normally used on absorbent materials, thus controlling drop deposition; conversely, when printed on non-absorbent substrate, such as plastics, the droplet is not easily blocked causing obvious print defects such as bleeding. By heating the target substrate, drop control could be improved as there would be a rapid evaporation of water and humectants. Unfortunately, since the target substrate is of a plastic nature, there would be deformations of the same. Various technologies have been proposed for the digital decoration of LVT floors, for example the US20180319148A1 patent provides for the use of a water-based ink and subsequent lamination.

Patent EP3095613B1 provides for the use of a water- based ink containing a polyurethane resin and subsequent lamination.

Patent US20200189250A1 provides for the use of a photo- crosslinkable ink on which an adhesive layer and subsequent lamination is applied. Patent WO2017017473A1 provides for the use of a photo- crosslinkable ink in which a vinyl resin is dissolved.

The aforementioned technologies, while providing a solution for the digital decoration of the LVT, do not overcome all the problems related to the same. Therefore, there is still a need for improved production methods of decorative surfaces using inkjet technology.

SUMMARY OF THE INVENTION The Applicant has discovered that the use of a decorative transfer method can solve the aforementioned problems.

The method object of the invention provides for the following steps: 1.Printing an ink on a transfer element 20 by means of inkjet printing (Fig. 1-2; Fig. 2- 1) .

2.Drying the ink (Fig. 1-3; Fig. 2-2). 3.Transfer the image 10 thus generated from the transfer element 20 to the substrate 11 to be decorated by heat and/or pressure (Fig. 1-4; Fig. 2-3). 4.Laminate the decorated layer with a thermoplastic film 12 by applying heat and/or pressure (Fig. 3).

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a schematic reproduction of the process of the invention of direct transfer to discrete target substrate mode.

Figure 2 illustrates a schematic reproduction of the process of the invention of indirect transfer to continuous target substrate mode.

Figure 3 illustrates the lamination and embossing of the decorated target substrate by means of a discontinuous press. GLOSSARY

TRANSFER ELEMENT: it's the element (e.g., belt, web, roll, cylinder) on which the image is printed and subsequently transferred on the target substrate.

TARGET SUBSTRATE: it's the material on which the printed image is transferred to.

LAMINATION LAYER: it's the thermoplastic layer which is laminated on the decorated TARGET SUBSTRATE. Usually, it's transparent and it's used to protect the decorated TARGET SUBSTRATE from abrasion. RADIATION CURABLE PRODUCT: (e.g., ink, coating, varnish, primer): radiation-cured products are formulated materials which are cross-linked or cured using high-intensity radiation energy from electron beams or ultraviolet light radiation.

UV CURABLE PRODUCT: same as RADIATION CURABLE PRODUCT PHOTOCURABLE PRODUCT: same as RADIATION CURABLE PRODUCT EMBOSSING: mould or stamp a design on a surface or object so that it stands out in relief.

TEXTURING: same as embossing.

THERMOPLASTIC: Denoting substances (especially synthetic resins) that become plastic on heating and harden on cooling and are able to repeat these processes.

DETAILED DESCRIPTION OF THE INVENTION

The method object of the invention provides for the printing of the image by means of inkjet on a material capable of transferring the image, henceforth "transfer element" and transferred to the material to be decorated, henceforth "target substrate", by pressure and/or temperature .

Depending on the target substrate, the transfer element can be in the form of continuous or discontinuous mode.

In a preferred form of the invention, the transfer element is an endless belt where the image 10 is printed by means of inkjet printing 2 and all colors are printed on the same belt. This preferred embodiment is illustrated in figure 1 in which the endless belt is indicated by numerals 20.

Pre-heating of the transfer element 20 can be provided as indicated in figure 1 by the preheating unit 1.

After printing a printing ink drying may be provided as also indicated in the preferred embodiment of figure 1 by the numerals 3.

The image 10 may be transferred directly on a surface of a substrate 11. In a variant embodiment, the image is transferred onto a white layer 13 which is applied to the substrate 11 before transferring the image 10 from the transfer element 20, in this case the endless belt to the white layer 13 on top of the substrate 11.

Substrate 11 with or without the white layer 13 is transported by a transport unit 30 such as the endless belt shown in figure 1 which supporting branch, in this case the upper branch is provided facing ta branch of the endless belt forming the transfer element 20, in this case the lower branch. The facing branches of the endless belts 30 and 20 being held at a distance which ensures the pressure needed for bonding the image to the substrate or to the white layer. The belts may be also provided in combination with heating means at least of the said facing or of at least one of the said facing branches for heating the image to be transferred and/or the substrate with or without the white layer to a bonging temperature.

In another form of the invention, the transfer element is in form of a cylinder 40. In this case there might be a single cylinder where the image is printed by means of inkjet printing and all colors are printed on the same cylinder. Alternatively, there might be several cylinders where only one color is printed on each cylinder. A combination of endless belt and cylinder might be used as well.

The transfer element 20, 40 must have the ability to completely transfer the image 10 onto the target substrate 11 without leaving residues on the transfer element 20, 40. Furthermore, the transfer element 20, 40 is preferably heat resistant up to a temperature of 150 ° C. In order to guarantee high print quality, the transfer element must be dimensionally stable at the temperature of use and has a surface tension sufficient to be printed by inkjet printing. In order to obtain a good printability, the difference between the surface tension of the transfer element and that of the ink should be at least 6 dynes/cm, better 8-10 dynes/cm. The following table shows the surface tension of the most common polymers which might be employed as transfer element:

Surface tension (dynes / cm) In one form of the invention, before the printing step a primer is applied to the transfer element which allows printability and/or transferability of the printed decoration. This material can be applied by means of traditional application techniques such as roller coater, spray coater, slot-die but it could also be applied digitally, e.g. by means of inkjet printing. As an example, the material could consist of a solution and/or dispersion of a thermoplastic resin. Typically inkjet printing involves the use of a printhead to create and jet droplets of liquid which will then form the image to be printed. As an example, details of this technology can be found in the book "Fundamentals of inkjet printing: the science of inkjet and droplets" (Hoath, Stephen).

Depending on the inkjet head used, the droplets produced can have different volumes and consequently different diameters. In addition to the native droplet size, an intrinsic characteristic of the head, larger droplets can be generated by the head itself. For example, a head capable of jetting 4 levels of gray will have the smallest drop of 6 pi while the largest will be 18 pi. Inkjet printing can be either in multipass/scanning mode where the image is generated with multiple passes of the head while the material to be printed advances or in singlepass mode, where the material to be printed passes only once under the heads which are installed at the width of the same material. Single-pass printing is used for large runs (> 1000 m2/h) while multipass printing, used for small and medium-sized runs (10-600 m2/h), is the most common.

In a preferred form of the invention, the image is printed on the heated transfer element. The temperature of the transfer element depends on the composition of the ink and is typically between 50°C and 150°C, preferably between 80°C and 120°C. By printing onto the heated transfer element, the water and humectant agents contained in the ink evaporate instantly thereby fixing the printed image. If it is necessary to increase evaporation, hot air and/or IR systems can be used after printing. The image is then transferred to the target layer to be decorated by means of pressure and/or heat. To achieve high printing quality, after transfer, the transfer element needs to be properly cleaned. In case of water-based inks, the transfer element might be cleaned by immersing it in water and afterwards dry it. In case of printing with radiation curable inks, the transfer element might be cleaned using an adhesive roller.

The transfer process might be direct or indirect. In case of direct transfer, the image is transferred directly from the transfer element to the target substrate. In case of indirect transfer, the image is transferred first to one or more additional transfer element which might be as an example an endless belt or a cylinder and then transferred to the target substrate.

In one form of the invention, the image is transferred on the target substrate continuously. In this mode the target substrate is in a web form, and it's continuously unwound and rewound.

In one form of the invention, the image is transferred on the target substrate discontinuously. In this case the target substrate is in discrete mode, such as a panel.

In case of LVT, the image might be transferred on white PVC film, the same as what is currently decorated by rotogravure printing. Typically, the film is white and made of unplasticized or low plasticized PVC and it has thickness of 70-90m. The film is then laminated to the core, which is generally made of PVC, typically a mixture of PVC, recycled PVC and calcium carbonate. The core might be flexible or rigid and it could also be made of other polymers like polyolefins, polyester, rubber and their combination. In principle the core could be also based of inorganic materials like calcium silicate, cement and the like. Usually all layers: core, decorated film and the protective layer are laminated and embossed in the same step. Alternatively, the PVC film might be already laminated to the core and the image is transferred to it. In this case the core is typically in form of panel and the transfer is made in discrete mode. In one form of the invention the ink is water-based and might contain water, humectants, pigments and additives to improve the wettability of the transfer element and / or the formation of the droplet.

In another form of the invention, the ink object of the invention is radiation curable (UV ink) and typically contains a mixture of monomers and/or oligomers, pigments, additives and photoinitiators.

In a preferred form of the invention, in order to increase the adhesion between the target substrate and the lamination layer the ink object contains dissolved and/or dispersed thermoplastic resin. The thermoplastic resins which might be used might consist, but not limited, of:

• PBI - polybenzimidazole

• PC - polycarbonate · PE - polyethylene

• PET - polyethylene terephthalate

• PMMA - polymethylmethacrylate

• PP - polypropylene

• PS - polystyrene · PVC - polyvinyl chloride

• PA - polyamide

• ABS - acrylonitrile butadiene styrene

• PLA - lactic polyacid

• PTFE - polytetrafluoroethylene · PES - polyethersulfone

• POM - polyoxymethylene

• PVAc - polyvinyl acetate

• EVA - ethylenevinylacetate copolymer • TPU - thermoplastic polyurethanes mixtures and copolymers of the above.

In a preferred form of the invention, the water-based ink preferably contains a dispersion of PVC and/or PVC-Ac copolymer and/or PVC-Ac-PVOH copolymer.While the photo- crosslinkable ink preferably contains dissolved and/or dispersed thermoplastic polymer consisting of PVC and/or PVC-Ac copolymer and/or PVC-Ac-PVOH copolymer.

In another form of the invention, after printing, an adhesion layer is applied to the printed image which favors adhesion with the subsequent lamination layer. The adhesion layer might be applied by means of traditional techniques such as roller coater, spray coater, slot-die or by means of inkjet printing. As an example, the adhesion layer might consist of a solution and/or dispersion of thermoplastic resin.

In a preferred embodiment of the invention, the decorated target layer 15 is laminated with a protecting layer of thermoplastic material 12. In a preferred form of the invention the lamination layer is transparent and it might be constituted, but not limited, of PVC, plasticized PVC, RET,RMMA, polyolefins and their copolymers.

Lamination can take place continuously using for example calenders, double belt presses, or in a discontinuous way using for example flat presses and membrane presses. Pressure, temperature and lamination time are a function of the materials to be laminated. As an example, in the case of PVC-based materials, lamination occurs at 170-200°C for about 1-2 minutes.

Normally, during lamination, a structured press element, such as press plate or a calander 50 is used to create the structure of the material to be reproduced, such as stones and woods. The press element is generally made of metal and the structure on it is made by mechanical and/or chemical processing.

In a preferred form of the invention, the structure, instead of being made with the aforementioned techniques, is made by means of additive digital printing, e.g. inkjet printing. In this way it is possible to produce just in time press elements obtaining the flexibility necessary for the decoration method object of the invention. In a preferred form of the invention the lamination layer is coated with a protective coating (top coating) to protect it from micro-scratches, scratches and chemical staining. Normally the coating is of the radiation curable type and contains fillers to increase scratch resistance. Example of the fillers are aluminum oxide, diamonds, silica, tungsten carbide, polyamide beads, HDPE beads. In another form of the invention the protective coating doesn't contain aluminum oxide.

Typically, the method object of the invention can be used for the production of floors, coverings, furniture, decorative profiles. A further application consists in the production of decorated laminating PVC film commonly used for furniture doors which have a structure very similar to LVT. The decorated laminating PVC film are commonly used to decorate MDF furniture parts such as kitchen doors. The decorated laminating PVC film might be 3D postformed by means of membrane press.

EXAMPLE 1 A printing system consisting of a transfer element and a single-pass printing unit with DIMATIX SG1024 heads was used. The printing speed was 30 m/min.

The transfer support made of TPU was preheated to 100°C by means of an IR lamp. Subsequently, an image consisting of a wood was applied to the transfer element using an ink with the following composition :

-Water: 69.4% -Propylene Glycol: 20%

-30% pigment black PBk 7 dispersion: 10%

-BYK 348: 0.6%

Thanks to the temperature of the transfer element, the ink is quickly fixed by evaporating water and glycol. Evaporation is then completed by subsequent IR heating.

In a subsequent phase, the image is transferred through the use of heat (100°C) and pressure (-1mm) on the panel to be decorated consisting of SPC (1000x1200x3.5mm) on which a white PVC film has been laminated (90m). Subsequently, a 300m transparent PVC film (wear layer) is applied to the surface of the panel in a static press equipped with a wood grain mold (180°C, 60"). At the end of the process, the panel is decorated with the wear layer embossed with woodgrain and well adhered to the surface of the panel.

EXAMPLE 2

A printing system consisting of a transfer element and a single-pass printing unit with DIMATIX SG1024 heads was used. The printing speed was 30 m/min.

The transfer support made of silicone was preheated to 100°C by means of an IR lamp.

Subsequently, an image consisting of a wood was applied to the transfer element using an ink with the following composition:

-Water: 59.4%

-Propylene Glycol: 20%

- 30% PVC dispersion in water: 10%

-30% pigment black PBk 7 dispersion: 10% -BYK 348: 0.6%

Thanks to the temperature of the transfer element, the ink is quickly fixed by evaporating water and glycol. Evaporation is then completed by subsequent IR heating. In a subsequent phase, the image is transferred through the use of heat (100°C) and pressure (-1mm) on the panel to be decorated consisting of SPC (1000x1200x3.5mm) on which a white PVC film has been laminated (90m).

Subsequently, a 300m transparent PVC film (wear layer) is applied to the surface of the panel in a static press equipped with a wood grain mold (180°C, 60"). At the end of the process, the panel is decorated with the wear layer embossed with woodgrain and well adhered to the surface of the panel.