Method for joining dissimilar surfaces using structured coating

Abstract

There is disclosed a method for joining a first and a second object comprising: a) applying an aqueous liquid comprising a compound comprising a carbon-carbon double bond on a surface of the first object, the first object comprises at least one polymer selected from the group consisting of poly(1-chloroethylene) (PVC), acrylonitrile butadiene styrene (ABS), and poly(1-methylethylene) (PP), wherein the aqueous liquid comprises a photoinitiator and a filler, wherein the filler comprises particles with a size 0.5 – 20 µm, wherein the aqueous liquid has a solid content of 5 - 80 wt%, b) evaporating at least a part of the aqueous liquid, c) irradiating the first object with actinic radiation, d) applying a glue on the first or second object or both, and joining the objects.

Description

[0001]STRUCTURED COATING Technical field The present invention relates to a method for joining dissimilar surfaces. Background Joining dissimilar materials with adhesives is a very common industrial process. In many cases low energy surfaces are bonded to high energy surfaces. This makes adhesive selection difficult, and in many instances a surface modification using a primer on one of the two surfaces is necessary for adequate bond strength. Applying edgebands to chipboard or medium density fibreboard surfaces are dominant in the furniture industry to provide the illusion of a homogeneous material. Edgebands can be made from many different materials: wood veneer, melamine paper and various thermoplastics. The latter need an adhesion primer to ensure solid bonding between the edgeband and wood surface. Many types of glues are used in the furniture industry, where hotmelts of three kinds dominate: Ethyl vinyl acetat (EVA), Polyolefins (PO) and Polyurethanes (PUR). The dissimilarity of these hotmelts in terms of chemistry and surface energy precludes the use of adhesion primers solely relying on surface energy matching by for example making the thermoplastic higher energy. Instead, solutions focusing on a structured primer surface to support adhesion via mechanical interlocking dominate. Typically, these primers rely on thermally activated isocyanate chemistry to attach to the edge band surface and organic solvents that create voids, valleys and peaks in the primer when evaporated. CN116606592 relates to the technical field of lubricating grease, in particular to a UV photocuring waterborne polyurethane primer for vehicles and a preparation method thereof. The UV photocuring waterborne polyurethane primer for the vehicle comprises the following raw materials in parts by weight: 15-28 parts of waterborne polyurethane resin, 2-5 parts of a waterborne silane coupling agent, 3-8 parts of an alkenyl phosphate adhesion promoter, 3-10 parts of alkenyl amine, 3-8 parts of waterborne pigment carbon black, 0.2-2 parts of a photoinitiator and 50-70 parts of deionized water. The UV photocuring waterborne polyurethane primer for the vehicle is safe, environmentally friendly, free of pollution, rapid in curing, high in flexible bonding force and hydrolysis resistance and wide in base material adaptability, and the bonding strength of polyurethane bonding glue and glass, aluminum materials, steel materials, baking varnish plates, plastic and other base materials difficult to permeate can be remarkably improved through the UV photocuring waterborne polyurethane primer; meanwhile, the system has relatively good storage stability and a good shelf life; the construction is simple and convenient, the influence on the body and the surrounding environment is avoided, and the bonding speed is high. US2003022005A1 describes a metal gasket raw material plate that is environmentally friendly and improves the peeling resistance between a metal plate and a rubber layer. The invention replaces traditional chromium-based treatments with a non-chromium approach that uses a primer layer composed of a silica-alumina condensate. This primer layer is chemically bonded to both the metal plate and the bonding layer, enhancing the adhesion and wear resistance of the rubber layer. The method involves applying a mixture of aluminum hydroxide and a silane coupling agent onto the metal plate, which is then heated to several hundred degrees to form the primer layer. CN108864854A belongs to the technical field of coating preparation and particularly discloses a single-component primer for ABS (acrylonitrile-Butadiene-Styrene) plastic and a preparation method of the single-component primer. The single-component primer for the ABS plastic is prepared from the following components in parts by weight: 40 to 60 parts of water-borne acrylic emulsion, 15 to 30 parts of deionized water, 5 to 10 parts of coalescing agent, 0.1 to 1 part of defoaming agent, 0.5 to 3 parts of leveling wetting agent, 5 to 10 parts of pH neutralizer, 3 to 8 parts of co-solvent, 0.5 to 3parts of thickening agent, 10 to 20 parts of water-borne graining paste, 0.5 to 2 parts of adhesion promoter and 5 to 10 parts of alkali wash mica powder. The single-component primer for the ABS plastic, prepared by the preparation method disclosed by the invention, has the advantages of good wear resistance, stable paint film, good heat and humidity resistance and capability of well meeting the needs of modern production and application. EP3935104B1 provides a method of coating a substrate comprising applying a first mixture where the first mixture reacts to form covalent bonds to the substrate surface and where the unreacted parts of the first mixture undergo diffusive mixing with a second layer, which is applied on top of the first mixture. This avoids creation of a weak layer, which may otherwise give lower adhesion. The adhesion as well as mechanical properties including the scratch resistance are improved. EP2643375B1 relates to a method for joining articles using aqueous UV-reactive primers for coating substrates based on ethylene vinyl acetate copolymers (EVA copolymers) or rubber. The invention improves the adhesion by combining ethylenically unsaturated monomers polymerized by UV light with polyurethane dispersions that have UV-polymerizable groups, forming primers that adhere well to substrates like EVA copolymers and rubber. The primers give an improved adhesion after curing. The document mentions substrates such as EVA with a vinyl acetate content ranging from 8 to 28 wt.%, particularly from 14 to 22 wt.%. It also lists various types of rubber, including styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), thermoplastic rubber (TR), natural rubber, and ethylene-propylene-diene rubber (EPDM), as well as polyolefins and other thermoplastic materials and mixtures thereof. Different additives are mentioned such as binders, auxiliary substances and additives known in coating and adhesives technology, in particular emulsifiers and light stabilisers, such as UV absorbers and sterically hindered amines (HALS), also antioxidants, fillers and auxiliary substances, for example antisettling agents, antifoams and / or wetting agents, flow improvers, reactive diluents, plasticisers, neutralising agents, catalysts, auxiliary solvents and / or thickeners, and additives such as, for example, pigments, colourings or mattifying agents and tackifiers. As fillers there are described mineral fillers, optionally also glass fibres, carbon blacks, nanoscale materials, such as silica sol or aluminium oxides or other metal oxides, and carbon nanotubes. Whereas for instance silica sol and aluminum oxides may be provided as particles, those fillers are described as a additives together with many other additives. Fillers are not described as increasing the adhesion. There is still a need for a primer for plastics that is easy to glue with various adhesives that do not form covalent bonds with the primer layer. Summary It is an object of the present invention to alleviate at least some of the disadvantages in the prior art and provide a method for laminating a printed substrate. In a first aspect there is provided a method for joining a first object and a second object, the method comprising the sequential steps: a. applying an aqueous liquid comprising at least one compound comprising a carbon-carbon double bond on a surface of the first object, the first object comprises at least one polymer selected from the group consisting of poly(1-chloroethylene) (PVC), acrylonitrile butadiene styrene (ABS), and poly(1- methylethylene) (PP), wherein the aqueous liquid comprises at least one photoinitiator, wherein the aqueous liquid comprises at least one filler, wherein the at least one filler comprises particles with a size measured according to ISO 13320:2020 in the range 0.5 – 20 µm, wherein the aqueous liquid has a solid content measured according to ISO 3251:2019 in the interval 5 wt% to 80 wt%, b. evaporating at least a part of the aqueous liquid on the surface of the first object, c. irradiating the first object with actinic radiation so that the photoinitiator initiates a chemical reaction involving the carbon-carbon double bond so that a covalent bond is formed, so that an irregular surface forms on the first object, d. applying a glue on at least one selected from the group consisting of the first object and the second object, and joining the first object and the second object. In a second aspect there is provided an object comprising a first object and a second object, wherein the first object and the second object are joined together with the method as described above. An advantage is that the adhesion is improved in particular for objects which otherwise are considered to be difficult to glue together. The method adds a surface roughness which contributes to improved adhesion for the glue. Detailed description Before the invention is disclosed and described in detail, it is to be understood that this invention is not limited to particular compounds, configurations, method steps, substrates, and materials disclosed herein as such compounds, configurations, method steps, substrates, and materials may vary somewhat. It is also to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting since the scope of the present invention is limited only by the appended claims. It must be noted that, as used in this specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. If nothing else is defined, any terms and scientific terminology used herein are intended to have the meanings commonly understood by those of skill in the art to which this invention pertains. As used herein (meth)acrylate is a general term that encompasses both acrylate and methacrylate. In the first aspect there is provided a method for joining a first object and a second object, the method comprising the sequential steps: a. applying an aqueous liquid comprising at least one compound comprising a carbon-carbon double bond on a surface of the first object, the first object comprises at least one polymer selected from the group consisting of poly(1-chloroethylene) (PVC), acrylonitrile butadiene styrene (ABS), and poly(1- methylethylene) (PP), wherein the aqueous liquid comprises at least one photoinitiator, wherein the aqueous liquid comprises at least one filler, wherein the at least one filler comprises particles with a size measured according to ISO 13320:2020 in the range 0.5 – 20 µm, wherein the aqueous liquid has a solid content measured according to ISO 3251:2019 in the interval 5 wt% to 80 wt%, b. evaporating at least a part of the aqueous liquid on the surface of the first object, c. irradiating the first object with actinic radiation so that the photoinitiator initiates a chemical reaction involving the carbon-carbon double bond so that a covalent bond is formed, so that an irregular surface forms on the first object, d. applying a glue on at least one selected from the group consisting of the first object and the second object and joining the first object and the second object. All three polymers (PVC, ABS, and PP) are considered to be difficult to join. This is due to a very low surface energy, in particular for PP. For PVC and ABS there may be a smooth surface which reduces bonding. Plasticizers in the substrate may reduce the efficiency for some adhesives, in particular for PVC. Substrates comprising any of those polymers would benefit from improved adhesion when joining with a glue. In one embodiment the first object comprises PVC. In one embodiment the first object comprises ABS. In one embodiment the first object comprises PP. In one embodiment the first object comprises at least one from PVC and ABS. In one embodiment the first object comprises at least one from PVC and PP. In one embodiment the first object comprises at least one from PP and ABS. In one embodiment the first object comprises PVC, PP and ABS. As described above a surface of the first object is treated to give an increased surface roughness. In an alternative embodiment both a surface of the first object and a surface of the second object are treated as described above to obtain an increased surface roughness. At least a part of the liquid on the surface is evaporated in step b). In one embodiment an amount of liquid is evaporated so that the surface appears dry, i.e. so that it does not look wet upon a visual inspection and so that no liquid is on the surface and can move on the surface. In one embodiment, the at least one compound comprising a carbon-carbon double bond comprises at least two carbon-carbon double bonds. In one embodiment, the at least one compound comprising a carbon-carbon double bond comprises at least three carbon-carbon double bonds. When the compound has several carbon-carbon double bonds, there are more bonds, which can react and give more covalent bonds after a reaction. Further, one molecule can be bound to several other molecules, which give a higher strength and more covalent bonds holding the structure together. In one embodiment, the aqueous liquid is mixed so that the at least one filler is added to the aqueous liquid after the at least one compound comprising a carbon-carbon double bond and the at least one photoinitiator. When the filler is added last or at least after the compound comprising a carbon-carbon double bond and the photoinitiator, then the mixing of the components of the aqueous liquid can become better. The aqueous liquid is referred to as a liquid, although it may comprise solid particles and / or only partially dissolved species. In this respect, also an aqueous dispersion and aqueous mixture are encompassed within the term aqueous liquid. The term aqueous is used because water is a major ingredient. A glue (also known as an adhesive) is a substance used to bond two or more surfaces together by forming an attachment at a molecular or mechanical level. Glues work by creating adhesion and cohesion (internal strength of the glue itself). Many different types of glue can be used in the present invention. In one embodiment, the glue comprises at least one compound selected from the group consisting of an ethyl vinyl acetate, a polyolefin, and a polyurethane. In another embodiment, the glue is a two-component epoxy glue. After step c) an irregular surface is formed. In one embodiment, the irregular surface on the first object, formed after step c) in claim 1 has a Rz value according to ISO 21920-2:2021 higher than 5 µm, preferably higher than 6 µm. In one embodiment the irregular surface on the first object, formed after step c) in claim 1 has a Ra value according to ISO 21920-2:2021 higher than 0.5 µm, preferably higher than 0.6 µm. In one embodiment, at least a part of at least one object selected from the group consisting of the first object and the second object is treated with at least one method selected from the group consisting of plasma treatment, corona treatment and flame treatment before step a). This can further improve the adhesion. In the second aspect there is provided an object comprising a first object and a second object, wherein the first object and the second object are joined together with the method as described above. Examples Example 1: In a 250 ml vial the following components were mixed; 5 g Glycerine propoxylate triacrylate (Laromer GPTA, BASF), 0.3g Speedcure 2-ITX (isopropyl thio xanthone, Sartomer), 0.3g Omnirad 1173 (2-hydroxy-2-methyl-1-phenyl-propan-1-one, IGM resins). The mixture was set in the oven at 70 °C for 10 minutes to allow the 2-ITX to dissolve. Then 15 g UCECOAT 7230 (a UV / EB energy curable polyurethane dispersion, Allnex), 75.8 g of deionized water was added to the mixture and stirred using a small lab mixer. While in the mixer, 20 g of Aktisil MAM (Methacrylate functionalized Neuburg Siliceous Earth, Hoffman minerals) was stirred in for 5 minutes. After this, 2 g Starch C*gel product S2(Starch from Cargill) was stirred in for 5 minutes. The mixture was applied to: a) a corona treated extruded polypropylene sheet and; b) an untreated extruded ABS sheet using at 12 g / m2using a 22 micron bar coater. The water was evaporated using a hand held hot air dryer for about 10 seconds, resulting in a film at around 6 microns. The primer layer was cured with 2 passes using a medium pressure mercury lamp at 4 m / min conveyor belt speed, where the total dose was 900 mJ / cm2UVA. The surface roughness of the samples were measured using a dektak profilometer from Mitutoyo. The samples were then glued using a 2 component expoxy to a melamine substrate composed of cured melamine paper on an MDF core board. A steel dolly was glued to the primed surface using Rapid hot melt EVA glue with a glue gun setting at 170 °C. The pull off force was recorded using an Elcometer 506 (a pull off adhesion tester from Elocmeter, UK). Briefly, a tool specifically made to grip the dolly was secured to the dolly and pressure was slowly increased, according to the instructions supplied by the manufacturer, until the dolly released from the substrate. The maximum pressure achieved was recorded and the mode of failure was inspected and recorded. Example 2: The same mixture as in example 1, except that the MAM was exchanged for Sphaeromer 6 (6 micron PMMA solid beads from Lamberti). The same application and tests as in example 1 were performed. Comparative example 1: The same mixture as in example 1, except that no filler was used. The same application and tests as in example 1 were performed. Comparative example 2: A standard parquet floor top coat (7710 from Bona) containing an acrylate monomer / oligomer acrylate mixture, fillers to reduce glossiness and various additives such as photoinitiators and levelling agents, was used. Here, since the formulation does not contain solvents, a 6 micron bar was used to provide a layer of similar thickness as the dried films in example 1 and 2 and comparative example 1. Since the formulation does not contain solvents, no evaporation was performed and hence there was no significant increase in surface roughness. The same tests as in example 1 were performed. Ra Rq Rz Pull off (micron) (micron) (micron (MPa) Example 1 1.35 1.61 7.89 >10* Example 2 0.86 1.15 6.62 >10* Comparative 0.41 0.53 2.90 2.74 example 1 Comparative 0.42 0.52 3.35 1.14 example 2 * Failed at a melamine substrate where the MDF cohesion was not sufficient to withstand the pull off pressure. The hot melt interface did not fail. From the examples above it is clear that the surface roughness effects bond strength significantly. The difference between comparative example 1 or 2 is likely to be additives used in the top coat for improved levelling and foam suppression, which results in worse adhesion to the hot melt glue. The surface roughness parameters Ra Rq Rz were as defined in ISO 21920-2:2021.

Claims

Claims 1. A method for joining a first object and a second object, the method comprising the sequential steps: a. applying an aqueous liquid comprising at least one compound comprising a carbon-carbon double bond on a surface of the first object, the first object comprises at least one polymer selected from the group consisting of poly(1-chloroethylene) (PVC), acrylonitrile butadiene styrene (ABS), and poly(1- methylethylene) (PP), wherein the aqueous liquid comprises at least one photoinitiator, wherein the aqueous liquid comprises at least one filler, wherein the at least one filler comprises particles with a size measured according to ISO 13320:2020 in the range 0.5 – 20 µm, wherein the aqueous liquid has a solid content measured according to ISO 3251:2019 in the interval 5 wt% to 80 wt%, b. evaporating at least a part of the aqueous liquid on the surface of the first object, c. irradiating the first object with actinic radiation so that the photoinitiator initiates a chemical reaction involving the carbon-carbon double bond so that a covalent bond is formed, so that an irregular surface forms on the first object, d. applying a glue on at least one selected from the group consisting of the first object and the second object, and joining the first object and the second object.

2. The method according to claim 1, wherein the at least one compound comprising a carbon-carbon double bond comprises at least two carbon-carbon double bonds.

3. The method according to claim 1, wherein the at least one compound comprising a carbon-carbon double bond comprises at least three carbon-carbon double bonds.

4. The method according to any one of claim 1-3, wherein the aqueous liquid is mixed so that the at least one filler is added to the aqueous liquid after the at least one compound comprising a carbon-carbon double bond and the at least one photoinitiator.

5. The method according to any one of claim 1-4, wherein the glue comprises at least one compound selected from the group consisting of an ethyl vinyl acetate, a polyolefin, and a polyurethane.

6. The method according to any one of claim 1-4, wherein the glue is a two-component epoxy glue.

7. The method according to any one of claim 1-5, wherein the irregular surface on the first object, formed after step c) in claim 1 has a Rz value according to ISO 21920- 2:2021 higher than 5 µm, preferably higher than 6 µm.

8. The method according to any one of claim 1-6, wherein the irregular surface on the first object, formed after step c) in claim 1 has an Ra value according to ISO 21920- 2:2021 higher than 0.5 µm, preferably higher than 0.6 µm.

9. The method according to any one of claim 1-7, wherein at least a part of at least one object selected from the group consisting of the first object and the second object is treated with at least one method selected from the group consisting of plasma treatment, corona treatment and flame treatment before step a) in claim 1.

0. An object comprising a first object and a second object, wherein the first object and the second object are joined together with the method according to any one of claims 1-9.