Preparation methods of embossed structures, embossed structures and their applications

Abstract

This invention relates to a method for preparing articles with embossed micron or nano-structured surfaces using an aqueous dispersion of a curable prepolymer, articles obtainable by said method, and uses of said articles.

Description

[0001] Invention Field

[0002] This invention relates to a method for preparing embossed micro- or nanostructured articles using an embossing lacquer formulation based on an aqueous dispersion, the structured articles obtained by said method, and the use of said structured articles. Background Technology

[0003] Due to the rapid rise of nanotechnology in recent years, the production of nanostructured components in industrial manufacturing has become increasingly important. Nanostructured components are typically made from photopolymerizable prepolymer compositions. The finest structures are increasingly used to achieve specific additional functions, such as self-cleaning surfaces, or even to create decorative visual effects or anti-reflective properties, particularly in security technology, decoration, product marketing, and surface treatment of various materials. For this purpose, structures processed using nanotechnology are used, made from special prepolymer compositions. One, and perhaps more importantly, purpose of these micron and nanostructured foils is for electronic, optical, sensing, and magnetic components, such as integrated circuits, displays, and micro-optics, because small structural dimensions are a decisive factor for the functionality of these items. Therefore, micron and nanostructured technologies (such as roll-to-roll, roll-to-plate, or sheet-to-sheet embossing techniques) play a crucial role in industrial foil manufacturing, and there is a need for continuously updated and improved moldable foils or compositions from which these structured foils can be manufactured.

[0004] One technique that enables the fabrication of structures in the nanometer range in parallel processes is called nanoimprint lithography (NIL), which provides a highly precise embossing process that can form even the smallest structures on the corresponding substrates.

[0005] Roll-to-roll nanoimprint lithography is already in industrial use. A crucial aspect of this method is preventing the embossing ink from adhering to the embossing tool, enabling subsequent defect-free demolding of the embossed object. Currently, two different types of embossing tools are used in roll-to-roll nanoimprint lithography: the embossing profile can be created either directly on the embossing roller or on a thin, flexible metal sheet or foil material firmly fixed around the roller. So-called nickel shims are used as these metal foils that can be arranged around the roller. Alternatively, an embossing stamp made of a polymer material can be used, where the surface energy of the polymer may be lower than that of nickel, reducing the adhesion of the embossing ink during the embossing process.

[0006] Embossed structures prepared from UV varnishes based on urethane acrylate (UA) oligomers are widely used because UA oligomers possess optimal mechanical and chemical stability. However, the high viscosity of UA oligomers makes the application of uniform films difficult.

[0007] Traditionally, in UV-NIL embossing paint formulations, acrylate monomers are added as reactive diluents to reduce the high viscosity of UA oligomers. However, acrylate monomers are generally irritating and hazardous, and because they cannot form hydrogen bonds, they adversely affect the mechanical and chemical stability of urethane acrylates.

[0008] EP 2 590 801 A1 discloses UV embossing paints comprising at least 60% by weight of an oligomeric component, a urethane acrylate oligomer, and 2 to 40% by weight of a reactive diluent. These paints are used for micron- and nano-structured surfaces with weather resistance and abrasion resistance.

[0009] UV varnishes for coatings other than embossed structures are increasingly based on aqueous UA dispersions, which allow for uniform application of thin films to substrates and provide extremely stable coatings after UV curing and / or crosslinking. The viscosity required for good application capability can be easily adjusted in aqueous UA dispersions.

[0010] The problem to be solved by the present invention

[0011] The existing technology does not disclose a method for using aqueous prepolymer dispersions as embossing paint.

[0012] Therefore, the fundamental problem of this invention is to provide a method for preparing micron or nanostructured articles using an environmentally friendly and low-toxicity embossing paint that is improved in terms of ease of viscosity adjustment without the use of reactive diluents. Summary of the Invention

[0013] The fundamental problem of this invention is solved by providing a method according to the invention. This invention also provides embossed articles and their uses.

[0014] The subject matter of this application is defined in the following embodiments [1] to

[15] and [1-1] to [14-3].

[0015] [1] A method for preparing an article having a micron or nanostructured surface, the method comprising the following steps in the order shown: (a) providing an aqueous dispersion of a prepolymer having at least one polymerizable carbon-carbon double bond, wherein the viscosity of the aqueous dispersion is less than 2000 mPa·s, (b) applying a film of the aqueous dispersion onto a surface of a substrate, (c) drying the film, (d) embossing the dried film to form a micron or nanostructure on the surface of the dried film, and (e) curing the embossed film to obtain an article having a micron or nanostructured surface.

[0016] [1-1] In the method of [1], step (d) of embossing the dried film is preferably performed without embossing the substrate.

[0017] [2] According to the method of [1] or [1-1], the prepolymer is selected from urethane acrylates, vinyl acrylates, epoxy acrylates, polyester acrylates, polyacrylates, polyether acrylates and polyolefin acrylates.

[0018] [2-1] In [2], urethane acrylate is preferred.

[0019] [3] The method according to any one of the foregoing embodiments, wherein the prepolymer comprises hydrophilic residues selected from acidic or ionic residues, preferably hydrophilic residues of non-acrylic acid residues.

[0020] [4] According to the method of [3], the hydrophilic residue is selected from carboxylic acid, sulfonic acid, carboxylate and sulfonate residues.

[0021] [4-1] Preferably, the features of [1] or [1-1] are combined with the features of [2-1] and [4].

[0022] [5] The method according to any one of the foregoing embodiments, wherein the weight-average molecular weight of the prepolymer is at least 2000 g / mol.

[0023] [6] The dried dispersion of the aqueous dispersion used in [1] or [1-1] preferably has a viscosity greater than 50 Pa·s. [6-1] More preferably, the combination of the features of [1] or [1-1] with the features of [5] and [6]. [6-2] Even more preferably, the combination of the features of [6-1] with the features of [4-1].

[0024] [7] The method according to any one of the foregoing embodiments, wherein, based on dry weight, it comprises 0% by weight to less than 2% by weight – preferably less than 0.1% by weight – a reactive diluent, said reactive diluent being a compound different from said prepolymer.

[0025] [8] The method according to [7], wherein the reactive diluent is a monomer.

[0026] [8-1] Preferably, the prepolymer has a weight-average molecular weight greater than 2000 g / mol, and the reactive diluent is a monomer with a weight-average molecular weight less than 300 g / mol.

[0027] [9] In the method according to any one of the foregoing embodiments, the aqueous dispersion comprises a surfactant, which is a compound different from the prepolymer and different from the optionally included reactive diluent, and is selected from alkyl-(meth)acrylates, polysiloxane (meth)acrylates, perfluoroalkyl (meth)acrylates, perfluoropolyether (meth)acrylates, alkyl vinyl ethers, polysiloxane vinyl ethers, perfluoroalkyl vinyl ethers, and perfluoropolyether vinyl ethers. In other words, the surfactant is selected from the group, while the prepolymer and the optionally included reactive diluent are not selected from the group.

[0028]

[10] The method according to [9], wherein, based on dry weight, it comprises 0.1% to 3% by weight of the surfactant.

[0029] [10-1] Preferably, the feature of [1] or [1-1] is combined with the features of [7] and

[10] . [10-2] More preferably, the feature of [6] is combined with the features of [10-1].

[0030]

[11] The method according to any one of the foregoing embodiments, wherein the dry material of the aqueous dispersion has a surface energy of 10 mN / m to 40 mN / m.

[0031] [11-1] Preferably, the combination of features of

[11] and [6]. [11-2] More preferably, the combination of features of

[11] and [10-1].

[0032]

[12] According to the method of [1] or [1-1], wherein the prepolymer is a urethane acrylate containing hydrophilic residues, wherein the hydrophilic residues are preferably non-acrylic residues, wherein the hydrophilic residues are selected from carboxylic acid, sulfonic acid, carboxylate and sulfonate residues, wherein the weight-average molecular weight of the prepolymer is at least 2000 g / mol, wherein the aqueous dispersion contains less than 2% by weight of monomer reactive diluent, and wherein the prepolymer is cured by UV radiation.

[0033] [12-1] Preferably, the combination of features of

[12] and [6]. [12-2] More preferably, the combination of features of

[12] and [10-1].

[0034]

[13] In any of the preceding embodiments, the number of double bonds of the prepolymer divided by the total mass of the prepolymer is less than 2.5 meq / g.

[0035] [13-1] Preferably, the feature of [1] or [1-1] is combined with the features of [5] and

[13] . [13-2] More preferably, the feature of

[12] and

[13] is combined.

[0036]

[14] Articles with micron or nano-structured surfaces can be obtained by the method of any one of the above embodiments.

[0037] [14-1] Preferably, the article can be obtained by the method described in

[12] ,

[13] , [13-1] or [13-2] and has a surface energy of less than 25 mN / m.

[0038] [14-2] The present invention also relates to articles having micron or nanostructured surfaces, said articles comprising a substrate and an embossed micron or nanostructured film on said substrate, said film comprising a cured UV prepolymer containing hydrophilic residues, said hydrophilic residues being selected from carboxylic acids, sulfonic acids, carboxylate residues, and sulfonate residues. The carboxylic acids and carboxylate residues are preferably not derived from acrylic acid.

[0039] [14-3] Preferably, the embossed curing film of the article described in

[14] , [14-1] or [14-2] comprises a polymer with an acid value of 5.0 to 30 mg KOH / g — preferably 10 to 30 mg KOH / g —.

[0040]

[15] One of the articles described above is used in sharkskin technology, moth eye technology, antifouling technology and refractive and superhydrophobic surface technology.

[0041] Furthermore, the subject matter of this application is defined in the following embodiments [1] to

[15] and [2-1] to [14-3].

[0042] [1] A method for preparing an article having a micron or nanostructured surface, the method comprising the following steps in the order shown: (a) providing an aqueous dispersion of a prepolymer having at least one polymerizable carbon-carbon double bond, (b) applying a film of the aqueous dispersion to a surface of a substrate, (c) drying the film, (d) embossing the dried film to form a micron or nanostructure on the surface of the dried film, and (e) curing the embossed film to obtain an article having a micron or nanostructured surface.

[0043] [2] According to the method of [1], the prepolymer is selected from urethane acrylates, vinyl acrylates, epoxy acrylates, polyester acrylates, polyacrylates, polyether acrylates and polyolefin acrylates. [2-1] In [2], urethane acrylates are preferred.

[0044] [3] The method according to any one of the foregoing embodiments, wherein the prepolymer comprises hydrophilic residues selected from acidic or ionic residues, preferably hydrophilic residues of non-acrylic acid residues.

[0045] [4] According to the method of [3], the hydrophilic residue is selected from carboxylic acid, sulfonic acid, carboxylate and sulfonate residues. [4-1] Preferably, a combination of the features of [1], [2-1] and [4] is preferred.

[0046] [5] The method according to any one of the foregoing embodiments, wherein the weight-average molecular weight of the prepolymer is at least 2000 g / mol.

[0047] [6] The method according to any one of the foregoing embodiments, wherein the viscosity of the aqueous dispersion is less than 2000 mPa·s. [6-1] The dried dispersion of the aqueous dispersion used in [6] preferably has a viscosity greater than 50 Pa·s. [6-2] More preferably, a combination of the features of [1], [5] and [6-1]. [6-3] Even more preferably, a combination of the features of [6-2] and [4-1].

[0048] [7] The method according to any one of the foregoing embodiments, wherein, based on dry weight, it comprises 0% by weight to less than 2% by weight – preferably less than 0.1% by weight – a reactive diluent, said reactive diluent being a compound different from said prepolymer.

[0049] [8] According to the method of [7], wherein the reactive diluent is a monomer. [8-1] Preferably, the prepolymer has a weight-average molecular weight greater than 2000 g / mol, and the reactive diluent is a monomer having a weight-average molecular weight less than 300 g / mol.

[0050] [9] In the method according to any one of the foregoing embodiments, the aqueous dispersion comprises a surfactant, which is a compound different from the prepolymer and different from the optionally included reactive diluent, and is selected from alkyl-(meth)acrylates, polysiloxane (meth)acrylates, perfluoroalkyl (meth)acrylates, perfluoropolyether (meth)acrylates, alkyl vinyl ethers, polysiloxane vinyl ethers, perfluoroalkyl vinyl ethers, and perfluoropolyether vinyl ethers. In other words, the surfactant is selected from the group, while the prepolymer and the optionally included reactive diluent are not selected from the group.

[0051]

[10] The method according to [9], wherein, based on dry weight, it comprises 0.1% to 3% by weight of the surfactant. [10-1] Preferably, a combination of the features of [1], [7] and

[10] . [10-2] More preferably, a combination of the features of [6-1] and [10-1].

[0052]

[11] According to any one of the preceding embodiments, the dry material of the aqueous dispersion has a surface energy of 10 mN / m to 40 mN / m. [11-1] Preferably, the combination of the features of

[11] and [6-1]. [11-2] More preferably, the combination of the features of

[11] and [10-1].

[0053]

[12] According to the method of [1], the prepolymer is a urethane acrylate containing hydrophilic residues, wherein the hydrophilic residues are preferably non-acrylic residues, the hydrophilic residues are selected from carboxylic acid, sulfonic acid, carboxylate and sulfonate residues, wherein the weight-average molecular weight of the prepolymer is at least 2000 g / mol, the aqueous dispersion has a viscosity of less than 5000 mPa·s, preferably less than 2000 mPa·s and contains less than 2% by weight of monomer reactive diluent, and wherein the prepolymer is cured by UV radiation.

[0054] [12-1] Preferably, the combination of features of

[12] and [6-1]. [12-2] More preferably, the combination of features of

[12] and [10-1].

[0055]

[13] According to the method of any one of [1] to

[12] , the number of double bonds in the prepolymer divided by the total mass of the prepolymer is less than 2.5 meq / g. [13-1] Preferably, a combination of the features of [1], [5], [6] and

[13] . [13-2] More preferably, a combination of the features of

[12] and

[13] .

[0056]

[14] Articles having micron or nanostructured surfaces can be obtained by the method of any one of the above embodiments. [14-1] Preferably, the article can be obtained by the method of

[12] ,

[13] , [13-1] or [13-2] and has a surface energy of less than 25 mN / m. [14-2] The present invention also relates to articles having micron or nanostructured surfaces, wherein the article comprises a substrate and an embossed micron or nanostructured film on the substrate, wherein the film comprises a cured UV prepolymer containing hydrophilic residues, wherein the hydrophilic residues are selected from carboxylic acid, sulfonic acid, carboxylate, and sulfonate residues. The carboxylic acid and carboxylate are preferably not derived from acrylic acid. [14-3] Preferably, the embossed cured film of the article of

[14] , [14-1] or [14-2] comprises a polymer with an acid value of 5.0 to 30 mg KOH / g—preferably 10 to 30 mg KOH / g.

[0057]

[15] One of the articles described above is used in sharkskin technology, moth-eye technology, antifouling technology and refractive and superhydrophobic surface technology.

[0058] Beneficial effects of the present invention

[0059] The use of aqueous dispersions allows for the adjustment of their properties. For example, viscosity can be easily adjusted by changing the water content of the aqueous dispersion. The aqueous dispersions used in this invention do not require reactive diluents that adversely affect the mechanical properties of the polymer. Therefore, the mixing ratio of the polymer to the reactive diluent can be selected from a wide range. Furthermore, the omission of reactive diluents makes the aqueous dispersions environmentally friendly and less toxic, and results in reduced tackiness of the embossing paint.

[0060] The aqueous dispersions used in this invention can be used as embossing paints, and are particularly well-suited for roll-to-roll, roll-to-plate, or sheet-to-sheet processes, exhibiting excellent molding properties that enable their use on an industrial scale. Due to their potentially extremely low surface energy and therefore extremely low adhesion energy, multiple generations of embossing and molding can also be performed without damage from embossing paint adhering to the embossing tools.

[0061] In addition, embossed paints with exceptional properties can be prepared, such as controllable mechanical properties (e.g., tensile strength, elongation at break, and modulus of elasticity) over a wide range, controllable surface energy (e.g., hydrophilicity or hydrophobicity and adhesion), high solvent and chemical resistance, high scratch and abrasion resistance, and high UV stability and weather resistance.

[0062] The embossed structure obtained by the method according to the invention can be used for micron-structured or nano-structured surfaces, for example, for reflective or hydrophobic surfaces. Detailed Implementation

[0063] A method for preparing articles with micron- or nano-structured surfaces includes the following steps in the order shown:

[0064] (a) An aqueous dispersion of a prepolymer having at least one polymerizable carbon-carbon double bond, wherein the viscosity of the aqueous dispersion is less than 2000 mPa·s.

[0065] (b) Applying a film of the aqueous dispersion to the surface of a substrate,

[0066] (c) Dry the membrane.

[0067] (d) Embossing the dried film to form micron or nanostructures on the surface of the dried film, and

[0068] (e) Curing the embossed film to obtain an article with a micron- or nano-structured surface. The articles of the present invention have a “micron- or nano-structured” surface. This means that the surface of the article contains depressions or protrusions in the micron range (i.e., 1 μm to less than 1000 μm) or in the nanon range (i.e., 1 nm to less than 1000 nm). The surface may contain both micron- and nano-structures.

[0069] A film (i.e., a thin layer) of the aqueous dispersion is applied to a substrate. The film can be applied with a thickness of 20 nm to 2000 µm, preferably 200 nm to 2000 µm, more preferably 2 µm to 1000 µm. The substrate can be made of any kind of material, such as glass, polymer, metal, wood, concrete, and stone. The substrate is preferably solid, or has a viscosity at least ten times that of the dried film in step (d). Preferably, this has the effect of not embossing the substrate when embossing the dried film in step (d). The aqueous dispersion is dried. The composition obtained by drying the aqueous dispersion used in this invention is sometimes simply referred to herein as a “dried dispersion.” The dried dispersion is an embossing paint that can be used for embossing in the method of this invention. Hereinafter, the terms “embossing paint” and “dried dispersion” are used synonymously. Water in the aqueous dispersion can be removed by heating and / or air convection. In this invention, the term “drying” covers the process of partially or completely removing water. The dried dispersion may contain a small amount of water or be anhydrous, for example, containing 0 to 5% by weight, preferably 0 to 2% by weight, of water. In one embodiment, the dried dispersion is anhydrous. In another embodiment, the dried dispersion may contain 0.1 to 5% by weight, of water. The embossing step is performed after the water content of the aqueous dispersion has decreased to a level that allows the film of the dispersion to have the desired viscosity and to be non-sticky. The substrate is preferably solid, or at least has a high viscosity. It is preferable to perform the embossing step so that the dried film is embossed while the substrate is not embossed. To achieve this selective embossing, the viscosity of the dried film, the viscosity of the substrate, and / or the embossing conditions can be appropriately adjusted. The thickness of the embossed film can be from 10 nm to 1000 µm, preferably from 100 nm to 1000 µm, more preferably from 1 µm to 500 µm. Curing is preferably carried out by UV radiation. In the case of UV radiation, the aqueous dispersion used in this invention contains a photoinitiator.

[0070] The micron or nanostructured surface of the article obtained in step (e) contains protrusions and depressions in the embossed film. Preferably, the ratio of the maximum thickness of the embossed film (i.e., the thickness at the top of the protrusion) to the minimum thickness of the embossed film (i.e., the thickness at the bottom of the depression) is at least 2. In other words, the embossed depressions are very deep relative to the total thickness of the embossed film. Therefore, the embossed film, which serves as a functional layer of the article, can remain relatively thin.

[0071] The embossed structure can be obtained by the method according to the invention. The embossed structure may contain water, for example, 0.1 to 5% by weight. If the water is not completely removed in step (c) and remains after curing, it can be removed after step (e). The article of the invention comprises a substrate and a cured polymer film having a micron or nanostructure on its surface. This article can be obtained by the method of the invention.

[0072] Articles are prepared using an aqueous dispersion of a prepolymer as a starting material. The dispersibility of the prepolymer in water requires a hydrophilic component, such as acidic or ionic residues in the prepolymer molecule. Hydrophilic components have adverse effects in non-aqueous (e.g., purely organic) compositions and are therefore not included in prepolymers of such compositions. Therefore, the aqueous dispersion and embossing paint used in this invention differ from conventional non-aqueous embossing paints. Consequently, the cured products obtained from the corresponding embossing paints also differ.

[0073] Due to the micron or nanostructure on the surface of the article of the present invention, the surface has specific properties that make the article usable in the following ways: sharkskin technology (for reducing viscous drag); moth-eye technology, such as moth-eye antireflective films for, for example, solar cells or glass surfaces; antifouling technology; refractive and superhydrophobic surface technology; and biomimetic surfaces, such as riblet foil for aircraft.

[0074] In this specification, compounds and components are generally described in the singular, such as "a" component or "containing a" component. It should be emphasized that, unless otherwise indicated, these formulations do not exclude the presence of more than one indicator component or compound. In this invention, references to standards and specifications refer to the most recent versions available at the time of filing of this application, unless otherwise indicated.

[0075] Aqueous dispersions

[0076] The aqueous dispersion used in the method according to the present invention may sometimes be simply referred to as "aqueous dispersion" in the following text.

[0077] The aqueous dispersion preferably comprises 10 to 90 wt% water and 90 to 10 wt% prepolymer, more preferably 40 to 80 wt% water and 20 to 60 wt% prepolymer, and even more preferably 50 to 70 wt% water and 30 to 50 wt% prepolymer. Preferably, the sum of the water content and the prepolymer content constitutes 80 to 100 wt% of the aqueous dispersion. In one embodiment, the aqueous dispersion comprises 50 to 70 wt% water and 30 to 50 wt% prepolymer, wherein the sum of the water content and the prepolymer content constitutes 80 to 100 wt% of the aqueous dispersion.

[0078] The dispersion is aqueous, i.e., water-based. It is preferably solvent-free unless otherwise explicitly stated. In some embodiments, the aqueous dispersion may contain a small amount of solvent, such as 0.01% to 10% by weight or 0.1% to 5% by weight.

[0079] The dispersion may contain other components, such as reactive diluents or photoinitiators. Photoinitiators may be selected from thioxanthone, ketone sulfone, (alkyl)benzoylphenylphosphine oxide, 1-hydroxyalkylphenyl ketone, or 2,2-dimethoxy-1,2-diphenylethane-1-one. The content of the photoinitiator may be from 0.1% to 10% by weight, particularly from 0.5% to 5% by weight. Other additives may be added to the curable precursor of the polymer material. These additives may provide certain desired properties to the polymer material contained in durable micron and / or nanostructured surfaces, or may allow for easier processing. Examples of such additives are colorants, rheology modifiers, leveling agents, anti-skin agents, UV stabilizers, antioxidants, and defoaming additives.

[0080] Furthermore, the present invention aims to use an aqueous dispersion to form a dried dispersion for use as an embossing paint.

[0081] Depending on the intended use, the viscosity of the aqueous dispersion can be adjusted by changing the content of water and / or reactive diluent. The dispersion preferably has a viscosity of 0.01 to 10 Pa·s, more preferably 10 to 2000 mPa·s, and most preferably 50 to 1000 mPa·s. Viscosities within this range result in good flowability of the aqueous dispersion and a uniform coating on the surface to be molded. Since the viscosity of the aqueous dispersion depends on its water content, the viscosity increases upon drying. In a preferred embodiment, the viscosity of the aqueous dispersion used in this invention is less than 10 Pa·s, while its dried viscosity is greater than 30 Pa·s. In a more preferred embodiment, the viscosity of the aqueous dispersion used in this invention is less than 2000 mPa·s, while its dried viscosity is greater than 50 Pa·s.

[0082] In this invention, viscosity is measured at 23°C according to DIN EN ISO 2555 (Brookfield method).

[0083] Since viscosity can be adjusted by changing the water content, reactive diluents are not required. Therefore, the aqueous dispersions used in this invention are preferably free of reactive diluents. However, reactive diluents may be included, for example, for adjusting the viscosity of the aqueous dispersion and / or the embossing paint. The dispersion may contain 0 to less than 2% by weight, preferably less than 0.1% by weight, and more preferably 0% by weight of the reactive diluents described below.

[0084] In one embodiment, the polymerizable component of the aqueous dispersion comprises: a prepolymer having at least one polymerizable carbon-carbon double bond, a component having at least two thiol groups (if contained), and a polymerizable surfactant (if contained). Therefore, reactive diluents having at least one polymerizable double bond are excluded from the aqueous dispersion.

[0085] The embossing paint (i.e., the dried dispersion) used in this invention has a surface energy of preferably 10 mN / m to 60 mN / m, more preferably 10 to 40 mN / m.

[0086] The composition and structure of an aqueous dispersion can be adjusted to enable the preparation of embossed and cured articles having a surface energy of less than 25 mN / m, wherein the surface energy is determined in a test method comprising the following steps: drying the aqueous dispersion; embossing the dried dispersion onto a nickel oxide surface, wherein the nickel oxide surface comprises a self-assembled fluorinated alkyl monolayer and has a surface energy of less than 15 mN / m; polymerizing the prepolymer; and determining the surface energy of the embossed surface of the polymer.

[0087] Examples of aqueous prepolymer dispersions and their preparation are disclosed in F. Masson, R. Schwalm, “UV-radiation curing of water based urethane–acrylate coatings,” Progress in Organic Coatings 39 (2000) 115–126. Water release during heating of the aqueous UV dispersion was shown to be temperature-dependent and film-thickness-dependent. Drying of a 30 μm thick coating of the aqueous UV dispersion was completed in less than 2 minutes when heated in an oven at 80 °C. Polymerization kinetics depended on parameters such as the type of photoinitiator, sample temperature, chemical structure of the functionalized oligomer, and its acid content. Water absorption was directly related to the carboxylic acid content of the urethane-acrylate polymer.

[0088] A series of UV-curable polyurethane coatings prepared by blending multifunctional thiol- and ene-terminated polyurethane aqueous dispersions are disclosed in Z. Yang, CE Hoyle, “Newly UV-curable polyurethanecoatings prepared by multifunctional thiol- and ene-terminated polyurethaneaqueous dispersions mixtures: Preparation and characterization,” Polymer 50 (2009) 1717–1722. The resulting polyurethane coatings exhibit good solution stability and high photopolymerization activity even after prolonged exposure. Incorporating water-based polyurethane chains into multifunctional thiol and ene monomers improves their solution stability and avoids any reactions between thiol and ene groups due to their high reactivity in non-aqueous systems. This method allows for the preparation of high-performance UV-curable polyurethane aqueous coatings based on a thiol-ene chemistry system.

[0089] Examples of aqueous dispersions that can be used in the methods according to the invention are disclosed in EP 2 288 638 B1. In a preferred embodiment of the invention, the method uses an aqueous dispersion comprising at least 90 wt% of a high molecular weight olefinically unsaturated polyurethane (A) and 0 to 5 wt% of one or more photoinitiators, wherein (A) can be obtained by reacting at least one polyisocyanate compound (Ai), at least one olefinically unsaturated compound (Aii), at least one hydrophilic compound (Aiii), optionally at least one chain extender containing active hydrogen (Avii), and optionally one or more polyols (Avi), wherein (Aii) comprises at least two reactive groups capable of reacting with isocyanate groups, and (Aiii) comprises at least one reactive group capable of reacting with isocyanate groups and enabling the polyurethane prepolymer to be dispersed in an aqueous medium either directly or after reacting with a neutralizing agent to provide a salt, wherein (Avii) is capable of reacting with isocyanate groups and providing chain growth. In this aqueous dispersion, the olefinically unsaturated polyurethane preferably has a viscosity of less than 2000 mPa·s and a weight-average molecular weight of at least 2000 g / mol. The polyisocyanate (Ai) can be selected from aliphatic and alicyclic polyisocyanates. The hydrophilic compound (Aiii) can be selected from the free general formula (HO). x R(COOH) yThe hydroxycarboxylic acid is represented by R, where R represents a straight-chain or branched hydrocarbon residue having 1 to 12 carbon atoms, and x and y are independently integers from 1 to 3. The olefinic unsaturated compound (Aii) is selected from the reaction product of a diglycidyl compound and (meth)acrylic acid, or from a diacrylate selected from bisphenol A diglycidyl ether. The chain extender containing active hydrogen (Avii) is selected from a polyamine having no more than 60 carbon atoms.

[0090] A particularly preferred aqueous dispersion comprises at least 90% by weight of a high molecular weight olefinically unsaturated polyurethane (A) and 0 to 5% by weight of one or more photoinitiators, wherein (A) has a viscosity of less than 2000 mPa·s and a weight-average molecular weight of at least 2000 g / mol, and is obtained by reacting at least one polyisocyanate compound (Ai), at least one olefinically unsaturated compound (Aii), at least one hydrophilic compound (Aiii), optionally at least one chain extender containing active hydrogen (Avii), and optionally one or more polyols (Avi), wherein (Ai) is selected from aliphatic and alicyclic polyisocyanates; (Aii) is selected from the reaction product of a diglycidyl compound with (meth)acrylic acid, or from a diacrylate of bisphenol A diglycidyl ether; and (Aiii) is selected from general formula (H0). x R(COOH) y The hydroxycarboxylic acid is represented by R, where R represents a straight-chain or branched hydrocarbon residue having 1 to 12 carbon atoms, and x and y are independently integers from 1 to 3; the (Avii) is selected from polyamines having no more than 60 carbon atoms.

[0091] prepolymer

[0092] The prepolymer contained in the aqueous dispersion used in the method according to the invention is water-dispersible and contains at least one polymerizable carbon-carbon double bond. It may be simply referred to herein as a "prepolymer". It can be cured (i.e., polymerized) to obtain a cured polymer.

[0093] Aqueous dispersions may contain a prepolymer as the sole polymerizable component, and therefore may not contain any polymerizable components other than the prepolymer. Alternatively, aqueous dispersions may contain a prepolymer and other copolymerizable components, such as chain extenders, which may be monomeric. In that case, the finished polymer is obtained, for example, by chain growth and by curing the polymerizable C-C double bonds. Chain growth can be carried out prior to curing and can be independent of curing. Preferably, chain growth and curing occur simultaneously. In any case, all components required to prepare the polymer are preferably contained in the aqueous dispersion, and no additional components need to be added for polymerization. In other words, the embossed and cured polymer is preferably obtained through a process consisting of reducing the water content of the aqueous dispersion, embossing the dried dispersion, and curing the embossed and dried dispersion.

[0094] The prepolymer can be cured by heating or radiation (i.e., treatment with ultraviolet light or electron beam). Preferably, the prepolymer is UV-curable. Free radical polymerization can be initiated by a photoinitiator. Before curing the prepolymer, water contained in the aqueous dispersion is removed, typically by heating.

[0095] The prepolymer is preferably an oligomer having at least two, more preferably at least five, or at least ten repeating units. The monomer units may be structurally identical or similar, or they may be different from each other.

[0096] Prepolymers with polymerizable C-C double bonds can be selected from acrylates, methacrylates, vinyl ethers, allyl ethers, propenyl ethers, olefins, dienes, unsaturated esters, allyl triazines, allyl isocyanates, and N-vinylamides.

[0097] In this invention, the terms "acrylate" or "acrylic acid" should be understood to include "(meth)acrylate" or "(meth)acrylic acid", respectively.

[0098] The aqueous dispersions used in this invention may contain, in addition to prepolymers containing C-C double bonds, components having at least two thiol groups. The double bonds of the prepolymer can react with the thiol-containing component to form a polymer. The component having at least two thiol groups is preferably a monomer having two thiol groups. Its content can be from 1 to 50% by weight, particularly from 5% to 30% by weight, and the prepolymer having at least one polymerizable double bond can be included in the aqueous dispersion in a total amount from 1% to 90% by weight, particularly from 40% to 80% by weight (based on dry weight).

[0099] In this article, the percentage "based on dry weight" refers to the percentage based on the content of solids (i.e., excluding water and other solvents, if present).

[0100] The prepolymer can be an oligomer or polymer with a weight-average molecular weight (Mw) of at least 2000 g / mol or even greater than 5000 g / mol. A lower Mw results in a lower viscosity. The selection of the functionality-related molecular weight of the oligomer ensures that the prepolymer flows into the stamp topography quickly enough, through which even very small structures can be safely and reliably molded.

[0101] The prepolymer contained in the aqueous dispersion is dispersible in an aqueous medium, preferably water. For this purpose, it possesses a certain degree of hydrophilicity. It preferably contains hydrophilic residues derived from the corresponding hydrophilic compound and enables the prepolymer to be dispersed in the aqueous medium either directly or after reacting with a neutralizing agent to provide a salt. The hydrophilic compound is generally selected from polyols containing functional groups that can exhibit ionic or nonionic hydrophilic properties. Preferred are polyols containing one or more anionic salt groups or acid groups that can be converted into anionic salt groups, such as carboxylate and sulfonate salt groups, and acid groups such as carboxylic acid or sulfonic acid groups. Preferably, it is a polyol of the general formula (HO). x R(COOH) y The term "hydroxycarboxylic acid" refers to a hydroxycarboxylic acid where R represents a straight-chain or branched hydrocarbon residue having 1 to 12 carbon atoms, and x and y are independently integers from 1 to 3. Examples of such hydroxycarboxylic acids include citric acid, malic acid, lactic acid, and tartaric acid. The most preferred hydroxycarboxylic acid is an α,α-dihydroxymethylalkanoic acid, where x=2 and y=1 in the above general formula, for example, 2,2-dihydroxymethylpropionic acid and 2,2-dihydroxymethylbutyric acid. Typically, aqueous dispersions require the initial neutralization of hydrophilic residues (e.g., carboxylic acid or sulfonic acid groups) provided by a hydrophilic compound to form a salt. This is typically accomplished by adding an organic or inorganic neutralizing agent, or a mixture thereof, to the prepolymer or water. Suitable neutralizing agents include: volatile organic tertiary amines, such as trimethylamine, triethylamine, triisopropylamine, tributylamine, N,N-dimethylcyclohexylamine, N,N-dimethylaniline, N-methylmorpholine, N-methylpiperazine, N-methylpyrrolidine, and N-methylpiperidine; and non-volatile inorganic bases comprising monovalent metal cations and anions, wherein the metal cations are preferably alkali metal ions such as lithium, sodium, and potassium ions, and the anions are, for example, hydroxide, hydride, carbonate, and bicarbonate ions, which are not retained in the aqueous dispersion itself. The total amount of these neutralizing agents can be calculated based on the total amount of acid groups to be neutralized. Typically, a stoichiometric ratio of approximately 1:1 is used.

[0102] Based on monomer units, the content of acid groups in the prepolymer containing the aqueous dispersion is preferably 0.5% to 3.0%, more preferably 1.0% to 2.5%. Alternatively, the acid value of the prepolymer is preferably 5.0 to 30 mg KOH / g, more preferably 10 to 30 mg KOH / g. Therefore, the articles of the present invention preferably have these acid contents.

[0103] The number of double bonds in the prepolymer divided by the total mass of the prepolymer is preferably less than 2.5 meq / g. Therefore, the crosslinking density of the cured polymer is low.

[0104] A typical example of the prepolymer used in this invention is a water-dispersible urethane acrylate oligomer.

[0105] Prepolymers can have additional polymerizable groups. For example, end-capped isocyanate groups can be used. The isocyanate groups are end-capped with protecting groups to prevent reaction with water in the dispersion. The protecting groups can be removed after drying the aqueous dispersion, and the isocyanate groups can react. Therefore, dual curing is possible. C. Decker, R. Schwalm, “Dual-Curing of Waterborne Urethane-Acrylate Coatings by UV and Thermal Processing,” Macromol. Mater. Eng. 2003, 288, 17–28, describes a water-based dual-cured urethane-acrylate oligomer synthesized by condensation polymerization of monomers containing hydroxyl, isocyanate, and acrylate groups. To obtain a stable aqueous dispersion, carboxyl groups are grafted onto the oligomer chain, and the isocyanate groups are protected with end-capping agents. After the water is released by brief heating, the dried film is cured by a combination of the following two methods: short UV exposure in the presence of a photoinitiator to induce polymerization of acrylate double bonds, and heating to no more than 150°C to release isocyanates and promote polycondensation by reaction with hydroxyl groups.

[0106] The prepolymers used in this invention are preferably selected from acrylates, methacrylates, vinyl ethers, allyl ethers, propenyl ethers, olefins, dienes, unsaturated esters, allyl triazines, allyl isocyanates, and N-vinylamides, and more preferably from acrylated oligomers or acrylated polymer compounds, such as urethane acrylates, vinyl acrylates, epoxy acrylates, polyester acrylates, polyacrylates, polyether acrylates, olefin acrylates, acrylated oils, and silicone acrylates. Urethane acrylates are particularly preferred.

[0107] The urethane acrylates used in this invention are preferably urethane acrylate oligomers. A particularly preferred class of urethane acrylate oligomers includes difunctional or higher-functional acrylates of hydroxyl-terminated NCO-extended polyesters or polyethers, which may be aliphatic or aromatic. The acrylated epoxy materials used in this invention are preferably acrylate epoxy oligomers. Acrylated epoxy oligomers include, for example, difunctional or higher-functional acrylates of epoxy resins, such as diacrylates of bisphenol A epoxy resins. Acrylated epoxy resins tend to improve the tensile strength and elongation at break of polymer materials obtained by curing UV-curable precursors. Polyacrylates are acrylic oligomers or polymers having reactive acrylic side groups or end groups capable of forming free radicals for subsequent reactions. Polyacrylate materials used in this invention are preferably polyacrylate polymers. Similar to acrylated epoxy materials, polyacrylates generally improve the tensile strength of materials obtained by curing UV-curable precursors. Similarly, acrylated olefin oligomers or polymers are unsaturated oligomers or polymers with reactive acrylic side groups or end groups capable of forming free radicals for crosslinking or chain growth. Acrylated olefins generally tend to improve the tensile strength and elongation at break of materials obtained by curing UV-curable precursors. Examples of usable acrylated olefins include polybutadiene acrylic oligomers. Suitable polyester acrylate oligomers or polymers in this invention are generally prepared by condensation reactions between acrylic acid and polyols or polyesters, respectively. Acrylated polyether resins tend to improve the flexibility and elongation at break of elastomeric materials obtained by curing UV-curable precursors. Silicone acrylate oligomers or polymers tend to improve the elasticity and elongation of structured surfaces, but compromise their tensile strength and robustness. Higher-functionality silicone acrylates are often used due to their low surface energy properties.

[0108] The mechanical properties of the resulting structured surface are influenced not only by the chemical composition of the curable oligomer or polymer, but also by their respective crosslinking densities. Higher crosslinking densities generally result in harder and more brittle materials, while lower crosslinking densities result in softer and more conformable materials.

[0109] reactive diluents

[0110] The dispersion may contain a reactive diluent. It is reactive because it is polymerizable and becomes part of the solidified polymer structure molecules; and it is a diluent because it reduces the viscosity of the dispersion. In particular, it reduces the viscosity of the dried dispersion. Preferably, the reactive diluent has at least one polymerizable double bond. The reactive diluent can be polymerized with the prepolymer used in this invention to form a copolymeric elastomeric network of the solidified polymer material in the structured surface of the article of this invention.

[0111] The reactive diluent preferably has a weight-average molecular weight (Mw) of less than 500 g / mol, and is therefore different from prepolymers with a weight-average molecular weight (Mw) greater than 2000 g / mol. In any case, the structure of the reactive diluent differs from that of the prepolymer and the surfactant (if present).

[0112] The reactive diluent may be selected from aliphatic (meth)acrylates or polyether (meth)acrylates, particularly HDDA or TMP(EO)xTA. Multifunctional aliphatic or polyether (meth)acrylates, particularly TMP(EO)9TA, TMP(EO)6TA, TMP(EO)3TA, or TMPTA, may be used; they are included as reactive diluents to specifically adjust the hardness of the polymer composition in addition to adjusting the viscosity of the aqueous dispersion and / or the dried dispersion.

[0113] surfactants

[0114] The aqueous dispersion used in this invention contains at least one surfactant, which acts as a surface-active anti-adhesive during the embossing process. In other words, the surfactant is a surface-active anti-adhesive.

[0115] Surfactants are compounds that differ from prepolymers and reactive diluents (if present) and have the effect of reducing the surface energy of dry materials. Therefore, the surface energy of dry materials containing surfactants is lower than that of dry materials without surfactants.

[0116] Typically, surfactants can be included in aqueous dispersions (i) as a component other than the prepolymer and / or (ii) as a component of the prepolymer molecules. In case (i), it is preferably polymerizable with the prepolymer during curing in step (e) of the method of the invention. Preferably, the surfactant has polymerizable carbon-carbon double bonds. In case (ii), it is structurally different from other components of the prepolymer.

[0117] In one embodiment, the aqueous dispersion used in this invention contains at least one surfactant selected from the group consisting of alkyl-(meth)acrylates, polysiloxane (meth)acrylates, perfluoroalkyl (meth)acrylates, perfluoropolyether (meth)acrylates, alkyl vinyl ethers, polysiloxane vinyl ethers, perfluoroalkyl vinyl ethers, and perfluoropolyether vinyl ethers. In this embodiment, the prepolymer and optionally contained reactive diluent are not selected from the group.

[0118] The surfactant may preferably be selected from at least one of the following curable surfactants: alkyl-(meth)acrylates, polysiloxane (meth)acrylates, perfluoroalkyl (meth)acrylates, and perfluoropolyether (meth)acrylates. The surfactant preferably contains a perfluoropolyether group or a fluoroalkyl group in its molecule, more preferably a perfluoropolyether group or a fluoroalkyl and (meth)acrylate combination. The surfactant may have a polysiloxane group or a perfluorocarbon group in its molecule.

[0119] Based on dry weight, the aqueous dispersion preferably contains 0.01% to 10% by weight, particularly 0.1% to 3% by weight, of a surfactant. This ensures that the surface energy of the prepolymer composition is reduced to a level where patterns can be repeatedly drawn multiple times by embossing or molding without defects.

[0120] Example

[0121] The present invention will be illustrated by the following examples.

[0122] (1) Examples of aqueous dispersions used in this invention

[0123] Table 1 illustrates the advantages of the aqueous dispersions used in this invention. These aqueous dispersions have low viscosity and therefore do not require the addition of monomers to reduce viscosity. The lower amount of double bond equivalent results in a lower degree of crosslinking after curing.

[0124] Table 1

[0125]

[0126] Resin:

[0127] A: Aliphatic trifunctional UA; B: Dipenta-pentaacrylate (16,000 mPa·s), Bisphenol A diacrylate (1,000 mPa·s), TPGDA-diethylamine (1 / 2) (20 mPa·s); C: Modified epoxy acrylate; D: Aliphatic trifunctional UA; E: Aliphatic difunctional UA; F: Aliphatic difunctional UA.

[0128] monomer:

[0129] THFA: Tetrahydrofurfuryl acrylate; HDDA: 1,6-hexanediol diacrylate; n-OA: n-octyl acrylate.

[0130] Other compositions that can be used in this invention are shown in Table 2 below.

[0131] Table 2

[0132]

[0133] (2) Preparation of articles according to the present invention

[0134] The embossed structure is prepared from an aqueous dispersion as follows:

[0135] - UCECOAT7700 (solvent-free aqueous UA oligomer dispersion) + 1% KL200 (photoinitiator) + 1% Additol VXW (leveling additive) + 1% RAD2200N (silicone acrylate)

[0136] - Melinex 506 membrane, 125 μm thick

[0137] - Bake at 100°C for 5 minutes; Result: Stickiness disappears.

[0138] - Molding by batch pressing with a nickel-riblet female stamp.

[0139] - Curing is performed using a UV desk lamp (wavelength: 365 nm), with light shining through the foil.

[0140] In another embodiment, good embossing results were achieved using the above-described aqueous dispersion formulation via an R2R-UV-NIL process at a web speed of 4 m / min. The aqueous dispersion formulation was applied to a Melinex 506 substrate web by R2R gravure printing, and its tackiness disappeared after a one-meter drying stretch at T = 100°C. Then, it was R2R embossed using a nickel roller pad and cured by UV light (λ = 365 nm) through the web.

[0141] The results show that the aqueous dispersion used in the method according to the present invention can be used to prepare embossed structures.

Claims

1. A method for preparing an article having a micron or nano-structured surface, the method comprising the following steps in the order shown: (a) An aqueous dispersion of a prepolymer having at least one polymerizable carbon-carbon double bond, wherein the viscosity of the aqueous dispersion is less than 2000 mPa·s. (b) Applying a film of the aqueous dispersion to the surface of a substrate, (c) Dry the membrane. (d) Embossing the dried film without embossing the substrate to form micron or nanostructures on the surface of the dried film, and (e) Curing the embossed film to obtain an article with a micron or nano-structured surface.

2. The method according to claim 1, wherein the prepolymer is selected from urethane acrylates, epoxy acrylates, polyester acrylates, polyacrylates, polyether acrylates and polyolefin acrylates.

3. The method according to any one of the preceding claims, wherein the prepolymer comprises a hydrophilic residue selected from acidic or ionic residues.

4. The method according to claim 3, wherein the hydrophilic residue is selected from carboxylic acids, sulfonic acids, carboxylate residues, and sulfonate residues.

5. The method according to any one of the preceding claims, wherein the weight-average molecular weight of the prepolymer is at least 2000 g / mol.

6. The method according to any one of the preceding claims, wherein, based on dry weight, the aqueous dispersion comprises 0% by weight to less than 2% by weight of a reactive diluent, said reactive diluent being a compound different from said prepolymer.

7. The method according to claim 6, wherein the reactive diluent is a monomer.

8. The method according to claim 6 or 7, wherein the aqueous dispersion comprises a surfactant, the surfactant being a compound different from the prepolymer and different from the reactive diluent, and selected from alkyl-(meth)acrylates, polysiloxane (meth)acrylates, perfluoroalkyl (meth)acrylates, perfluoropolyether (meth)acrylates, alkyl vinyl ethers, polysiloxane vinyl ethers, perfluoroalkyl vinyl ethers, and perfluoropolyether vinyl ethers.

9. The method of claim 8, wherein the aqueous dispersion comprises 0.1% to 3% by weight of the surfactant based on dry weight.

10. The method according to any one of the preceding claims, wherein the dry material of the aqueous dispersion has a surface energy of 10 mN / m to 40 mN / m.

11. The method of claim 1, wherein the prepolymer is a urethane acrylate containing hydrophilic residues, wherein the hydrophilic residues are selected from carboxylic acid, sulfonic acid, carboxylate, and sulfonate residues, wherein the weight-average molecular weight of the prepolymer is at least 2000 g / mol, the aqueous dispersion contains less than 2% by weight of a monomeric reactive diluent, and wherein the prepolymer is cured by UV radiation.

12. The method according to any one of claims 1 to 11, wherein the number of double bond equivalents of the prepolymer divided by the total mass of the prepolymer is less than 2.5 meq / g.

13. Articles having micron or nanostructured surfaces that can be obtained by the method of any one of claims 1 to 12.

14. Use of the article having a micron or nanostructured surface as described in claim 13 in sharkskin technology, moth-eye technology, and diffractive or refractive light guiding and superhydrophobic surface technology.

15. Use of the article having a micron or nano-structured surface as described in claim 13 in antifouling technology.

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