Optically transparent pressure-sensitive adhesive, adhesive sheet and use thereof

Abstract

The present invention relates to an optically transparent pressure sensitive adhesive having a dynamic shear strength of at least 1 MPa as measured according to ASTM D1002 and having a delta b value of less than 2 as measured according to EN ISO 11664 or a yellowing index of less than or equal to 1.5 as measured according to ASTM E313 after storage at 105 DEG C for 1000 hours. The invention further relates to an adhesive sheet comprising the optically transparent pressure-sensitive adhesive and to the use of the optically transparent pressure-sensitive adhesive or of the adhesive sheet for bonding two substrates.

Description

Technical Field

[0001] This invention relates to an optically transparent pressure-sensitive adhesive, an adhesive sheet comprising the optically transparent pressure-sensitive adhesive, and the use of the optically transparent pressure-sensitive adhesive or adhesive sheet for bonding two substrates. Background Technology

[0002] The joining of individual components is one of the core methods of manufacturing technology. Besides methods such as welding and brazing, an important method used today for joining individual components is adhesive bonding, that is, bonding using adhesives, especially sheet-like adhesives such as tapes. Pressure-sensitive adhesives are known for their use in manufacturing and everyday life; they are persistently tacky and adhesive under normal environmental conditions, and can be easily applied to and adhered to a substrate by means of pressure, but can later be removed from the substrate more or less without residue. These adhesives are widely used in various fields such as machinery, electronics, automobiles, and construction due to their properties.

[0003] The diversification of products has led to numerous applications for bonding curved surfaces, particularly in the electronics, automotive, and construction industries. Examples include bonding displays, smart car windows, and architectural glass. Glass used in these applications is often not perfectly flat but frequently features 3D curved surfaces, placing stringent demands on adhesives. On one hand, bonding this type of glass to the substrate requires adhesives with sufficient thickness to compensate for gaps caused by irregular bonding surfaces, facilitating easy adherence to curved surfaces. Simultaneously, the adhesive must exhibit low yellowing properties (typically a Δb value less than 2 or a yellowing index less than or equal to 1.5), especially to overcome yellowing as adhesive thickness increases. On the other hand, the adhesive needs to possess high optical transparency and low haze (to prevent whitening and cloudiness), meeting the good optical properties required of conventional optically transparent pressure-sensitive adhesives. In addition, and more importantly, it must possess superior mechanical and adhesive properties in such applications, so that it can absorb the stress generated by bending when bonding 3D curved substrates and prevent the edge of the curved surface from rebounding and delaminating. In other words, it should not produce defects or delamination after bonding, and should still maintain good adhesion even under high temperature and high humidity conditions.

[0004] When compensating for gaps caused by irregular bonding surfaces, pressure-sensitive adhesives are typically applied at a greater thickness. In such cases, the adhesive applied at a greater thickness is affected by the usage environment, such as hot and humid outdoor weather, and over time, this can cause the bonded products to lose their satisfactory appearance (e.g., bubbling, yellowing, whitening, etc.), and sometimes even lead to separation of the bonded products.

[0005] Therefore, there remains a need for adhesives that can maintain good adhesion and appearance for extended periods when bonding products to curved substrates, possess superior mechanical and adhesive properties, do not rebound or delaminate, and also have good optical properties, especially exhibiting low yellowing and good durability when manufactured in thicker thicknesses. Summary of the Invention

[0006] The purpose of this invention is to provide an optically transparent pressure-sensitive adhesive that can maintain good adhesion and appearance of the bonded product for a long time, while possessing good optical properties (in particular, low yellowing properties and good optical transparency), as well as superior mechanical and adhesive properties.

[0007] Furthermore, an object of the present invention is to provide an adhesive sheet comprising the aforementioned optically transparent pressure-sensitive adhesive, which is particularly suitable for bonding substrates with 3D curved surfaces. On one hand, the sheet can have a thickness of not less than 250 μm, for example not less than 500 μm, or even 1000 μm, thereby adequately compensating for voids caused by irregular 3D curved surfaces. On the other hand, the sheet, at such a high thickness, can simultaneously exhibit low yellowing properties and good optical transparency. The sheet can maintain good adhesion and a good appearance of the product even under prolonged use in humid and hot conditions.

[0008] Accordingly, the object of the present invention is to provide the use of the optically transparent pressure-sensitive adhesive or the adhesive sheet for bonding two substrates, preferably wherein at least one substrate is a 3D curved surface.

[0009] Therefore, in a first aspect, the present invention provides an optically transparent pressure-sensitive adhesive having a dynamic shear strength of at least 1 MPa, preferably 1 MPa to 3 MPa, as measured according to ASTM D1002, and a yellowing index of less than 2 as measured according to EN ISO 11664, or less than or equal to 1.5 as measured according to ASTM E313, after being stored at 105°C for 1000 hours. In particular, when the optically transparent pressure-sensitive adhesive is formed into a sheet with a thickness of not less than 500 μm, particularly 1000 μm, it still has a dynamic shear strength of at least 1 MPa, preferably 1 MPa to 3 MPa, as measured according to ASTM D1002, and a yellowing index of less than 2, or less than or equal to 1.5, after being stored at 105°C for 1000 hours.

[0010] Secondly, the present invention provides an adhesive sheet comprising an optically transparent pressure-sensitive adhesive according to a first aspect of the invention, said adhesive sheet having a thickness greater than or equal to 250 μm, preferably greater than 500 μm, more preferably 500 to 1000 μm. The sheet exhibits low yellowing properties and good optical transparency even at a relatively high thickness. It also maintains good adhesion and a good appearance of the product for a long time under humid and hot conditions.

[0011] Thirdly, the present invention provides the use of an optically transparent pressure-sensitive adhesive according to the first aspect of the invention or an adhesive sheet according to the second aspect of the invention for bonding two substrates, preferably, one substrate is 3D curved and the other substrate is planar, single-curved or 3D curved. Attached Figure Description

[0012] The above and other aspects, features, and advantages of certain embodiments of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, wherein:

[0013] Figure 1 A schematic diagram showing the dynamic shear strength test; and

[0014] Figure 2 This diagram illustrates the anti-rebound test. Detailed Implementation

[0015] The above objectives are achieved through the subject matter defined according to the invention. Preferred designs according to the invention arise from further extensions of the invention and the following embodiments.

[0016] The preferred embodiments referred to below can be combined with features of other embodiments referred to as preferred embodiments in particularly preferred embodiments. Therefore, combinations of two or more embodiments referred to as particularly preferred embodiments are very particularly preferred. It is also preferred that features of one embodiment referred to in some respects as preferred are combined with one or more other features of another embodiment referred to in some respects as preferred. Therefore, combinations of various features are included in the invention, and in this case, different levels of preference are also included. Thus, for example, a combination of a first feature referred to as “preferred” and a second feature referred to as “particularly preferred” is included in the invention. Here, the features of the preferred adhesive sheet and its use derive from the features of the preferred pressure-sensitive adhesive. The features of the preferred pressure-sensitive adhesive and its use also derive from the features of the preferred adhesive sheet.

[0017] As is known, pressure-sensitive adhesives according to the present invention should be understood to refer—particularly at room temperature—to substances with specific and lasting tack and adhesiveness. Pressure-sensitive adhesives are characterized by their ability to adhere to a substrate by applying pressure, without needing to define in detail the applied pressure and the duration of that pressure. In some cases, depending on the exact properties of the pressure-sensitive adhesive, temperature and humidity, and the substrate, a minimal pressure applied for a short period of time with gentle contact may be sufficient to achieve adhesion; in other cases, higher pressure and a longer application time may be necessary.

[0018] Pressure-sensitive adhesives possess specific characteristic viscoelastic properties, resulting in durable adhesion. Their characteristic is that, upon mechanical deformation, both a viscous flow process and the formation of an elastic restoring force occur. These two processes are specifically related to each other in their respective proportions, depending not only on the precise composition, structure, and degree of cross-linking of the pressure-sensitive adhesive material, but also on the rate and duration of deformation, and on the temperature.

[0019] A certain proportion of viscous flow is necessary for adhesion. The viscous component, generated solely by macromolecules with relatively high mobility, allows for effective wetting of the substrate to be bonded and efficient flow onto it. High viscous flow components result in high pressure-sensitive adhesive tack (also known as surface tack) and therefore often also high adhesive strength. Due to the lack of flowable components, highly cross-linked systems, crystalline or glassy cured polymers generally exhibit only very low pressure-sensitive adhesive tack or none at all.

[0020] A certain proportion of elastic restoring force is necessary for the realization of cohesion. These forces are generated, for example, by macromolecules with very long and highly coiled chains and by physical or chemical cross-linking, and allow the transmission of forces acting on the adhesive bond. This results in the adhesive bond being able to withstand long-term loads acting on it, such as in the form of long-term shear loads, for a relatively long period of time.

[0021] To more accurately describe and quantify the measures of elastic and viscous components, and the relationship between them, variables that can be determined by dynamic mechanical analysis (DMA, according to DIN EN ISO 6721) can be used: storage modulus (G′) and loss modulus (G″). G′ is a measure of the elastic component of the material, and G″ is a measure of the viscous component. Both variables depend on the deformation frequency and temperature.

[0022] The variable can be measured using a rheometer. Here, for example, the material under study is exposed to sinusoidal oscillating shear stress in a plate-plate arrangement. In the case of an instrument operating under shear stress control, the deformation is measured as a function of time, and the time shift of this deformation relative to the introduction of shear stress is measured. This time shift is called the phase angle δ.

[0023] The storage modulus G′ is defined as follows: G′=(τ / γ)·cos(δ) (τ=shear stress, γ=deformation, δ=phase angle=phase shift between the shear stress vector and the deformation vector). The loss modulus G″ is defined as follows: G″=(τ / γ)·sin(δ) (τ=shear stress, γ=deformation, δ=phase angle=phase shift between the shear stress vector and the deformation vector).

[0024] If at room temperature, defined here as 23°C, in 10 0 -10 1 The deformation frequency range in rad / second (radians / second), where G′ is at least partially located within 10 3 -10 7 A substance is generally considered pressure-sensitive adhesive if it is within the range of Pa, and if G′ is also at least partially within that range, and is defined as pressure-sensitive adhesive in the sense of this invention. “Partially” means that at least a portion (at least a segment) of the G′ curve lies within the range from 10... 0 rad / s (including endpoints) to 10 1 The range of deformation frequencies in rad / s (including endpoints) (x-axis) and from 10 3 (Including endpoints) Pa to 10 7 The range of G′ values ​​(vertical axis) across Pa (including endpoints) is within the window spanned. This applies accordingly to G″.

[0025] The optically transparent pressure-sensitive adhesive according to the first aspect of the invention has a dynamic shear strength of at least 1 MPa, preferably 1 MPa to 3 MPa, as measured according to ASTM D1002, and has a Δb value of less than 2 as measured according to EN ISO 11664, or a yellowing index of less than or equal to 1.5 as measured according to ASTM E313 after 1000 hours of storage at 105°C.

[0026] The Δb value refers to the change in sample color along the yellow-blue axis (b-axis) in color measurement. It is commonly used to indicate the degree to which a material or product yellows under the influence of aging, light exposure, or other environmental factors. A larger Δb value indicates a more pronounced yellowing. The thickness of optically transparent adhesives affects their yellowing Δb value. Generally, as the adhesive thickness increases, the yellowing Δb value also increases. This is because a thicker adhesive layer absorbs and scatters more light, resulting in a more noticeable color change. Therefore, to ensure that the optical performance of the final product meets requirements, the impact of thickness on yellowing must be considered. In a preferred embodiment of the invention, when the optically transparent pressure-sensitive adhesive is made into a sheet with a thickness of not less than 500 μm, particularly 1000 μm, it still has a dynamic shear strength of at least 1 MPa, preferably 1 MPa to 3 MPa, as measured according to ASTM D1002, and has a Δb value of less than 2 as measured according to EN ISO11664, or a yellowing index of less than or equal to 1.5 as measured according to ASTM E313, after being stored at 105°C for 1000 hours.

[0027] In embodiments of the present invention, the dynamic shear strength may be at least 1 MPa, preferably 1 MPa to 3 MPa, 1 MPa to 1.7 MPa, 1.2 MPa to 1.5 MPa, or 1.3 MPa to 1.4 MPa. For example, when the optically transparent pressure-sensitive adhesive is made into a sheet with a thickness of 500 μm, the dynamic shear strength may be at least 1.3 MPa, for example greater than 2 MPa, 1.3 MPa to 1.7 MPa, or 1.5 MPa. For example, when the optically transparent pressure-sensitive adhesive is made into a sheet with a thickness of 1000 μm, the dynamic shear strength may be at least 1 MPa, for example 1 MPa to 1.7 MPa, or 1.2 MPa to 1.4 MPa. When the pressure-sensitive adhesive has a dynamic shear strength within the above range, the adhesive can maintain the adhesion and good appearance of the bonded product for a long time.

[0028] In embodiments of the present invention, the Δb value may be less than 2, for example less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.2, less than or equal to 1, less than or equal to 0.8, less than or equal to 0.6, or less than or equal to 0.5, for example, 1 to 1.6, or 0.5 to 0.8. For example, when the optically transparent pressure-sensitive adhesive is made into a sheet with a thickness of 500 μm, the Δb value may be less than or equal to 1, less than or equal to 0.8, less than or equal to 0.6, or less than or equal to 0.5, for example, 0.5 to 0.8, or 0.6 to 0.7. For example, when the optically transparent pressure-sensitive adhesive is made into a sheet with a thickness of 1000 μm, the Δb value may be less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.2, or less than or equal to 1, for example, 1 to 1.6, or 1.2 to 1.4. In embodiments of the present invention, the yellowing index may be less than or equal to 1.5, for example, less than or equal to 1.3, less than or equal to 1.1, less than or equal to 1, less than or equal to 0.8, less than or equal to 0.7, or less than or equal to 0.6, for example, 1 to 1.5, or 0.6 to 0.8. For example, when the optically transparent pressure-sensitive adhesive is made into a sheet with a thickness of 500 μm, the yellowing index may be less than or equal to 0.8, less than or equal to 0.7, or less than or equal to 0.6, for example, 0.6 to 0.8, or 0.6 to 0.7. For example, when the optically transparent pressure-sensitive adhesive is made into a sheet with a thickness of 1000 μm, the yellowing index may be less than or equal to 1.5, less than or equal to 1.3, less than or equal to 1.1, or less than or equal to 1.07, for example, 1 to 1.3, or 1 to 1.1. When the pressure-sensitive adhesive has a Δb value or yellowing index within the above range, the adhesive has good optical properties.

[0029] In a preferred embodiment of the invention, the optically transparent pressure-sensitive adhesive, after being stored at 85°C and 85% relative humidity for 1200 hours, has a haze value less than or equal to 1, preferably less than or equal to 0.5, more preferably less than or equal to 0.2, and even more preferably less than or equal to 0.1, and particularly 0, as measured according to ASTM D1003. For example, when the optically transparent pressure-sensitive adhesive is formed into a 500 μm thick sheet, the haze value may be less than or equal to 0.4, less than or equal to 0.3, or less than or equal to 0.1, for example, 0 to 0.3, or 0 to 0.1. For example, when the optically transparent pressure-sensitive adhesive is formed into a 1000 μm thick sheet, the haze value may be less than or equal to 0.7, less than or equal to 0.6, less than or equal to 0.4, or less than or equal to 0.2, for example, 0 to 0.6, or 0 to 0.2.

[0030] In a preferred embodiment of the invention, the optically transparent pressure-sensitive adhesive has a refractive index greater than or equal to 1.510, for example 1.510 to 1.580, 1.515 to 1.580, 1.510 to 1.550, 1.510 to 1.540, 1.510 to 1.530, or 1.510 to 1.520 at a wavelength of 589 nm.

[0031] In embodiments of the present invention, the optically transparent pressure-sensitive adhesive may be obtained from a monomer composition comprising:

[0032] a) at least one hydroxyl-containing acrylate monomer, at least 25% by weight, preferably 25 to 45% by weight, more preferably 30 to 35% by weight;

[0033] b) At least one high-refractive-index monomer, at least 30% by weight, preferably 30 to 45% by weight, more preferably 35 to 40% by weight or 30 to 35% by weight;

[0034] c) at least one high-Tg monomer comprising 5% or more by weight, preferably 8 to 40% by weight, more preferably 10 to 30% by weight; and

[0035] d) Optionally, at least one monomer that is different from monomers a), b) and c) and can copolymerize with monomers a), b) and c), based on the total weight of the monomer composition.

[0036] In an embodiment of the present invention, the hydroxyl-containing acrylate monomer a) may be a hydroxy C1-C6 alkyl ester of (meth)acrylate, preferably selected from 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, more preferably selected from 4-hydroxybutyl acrylate (4-HBA) and 2-hydroxyethyl acrylate (2-HEA), and more preferably 4-hydroxybutyl acrylate (4-HBA).

[0037] Within the scope of this invention, the term "(meth)acrylic acid" is intended to represent acrylic acid and / or methacrylic acid. Therefore, hydroxy C1-C6 alkyl esters of (meth)acrylic acid correspondingly encompass hydroxy C1-C6 alkyl esters of acrylic acid and / or hydroxy C1-C6 alkyl esters of methacrylic acid.

[0038] In embodiments of the invention, a high-refractive-index monomer is understood to be a monomer having a refractive index greater than or equal to 1.510 at a wavelength of 589 nm and particularly containing at least one aromatic ring structure. Preferably, the high-refractive-index monomers used according to the invention may have a refractive index of 1.510 to 1.580, preferably 1.515 to 1.580.

[0039] In recent years, with the diversification of optical components, refractive index has become an important performance characteristic. Examples of applications using adhesives with high refractive indices include hybrid aspherical lenses, dichroic lenses, prisms, architectural glass, and intelligent automotive windows, all made of glass-resin composites. Adhesives used in these applications require high refractive indices. In principle, the monomers with high refractive indices that can be used in this invention are not particularly limited, as long as they have the aforementioned refractive index and specifically contain at least one aromatic ring structure. By using monomers containing these aromatic ring structures in the monomer compositions of this invention, the refractive index of the resulting adhesive layer can be increased.

[0040] In embodiments of the invention, the high-refractive-index monomer b) may be a (meth)acrylate containing at least one aromatic ring, preferably selected from benzyl acrylate (BZA), phenoxybenzyl acrylate (PBA), 2-phenoxyethyl acrylate (2-PEA), and o-phenylphenoxyethyl acrylate (OPPEA), with benzyl acrylate (BZA) and phenoxybenzyl acrylate (PBA) being more preferred. When these monomers are included in the monomer composition forming the pressure-sensitive adhesive of the invention, the pressure-sensitive adhesive of the invention advantageously possesses particularly excellent optical properties, especially a high refractive index greater than or equal to 1.510 at a wavelength of 589 nm.

[0041] In embodiments of the invention, a high Tg monomer is understood to be a monomer having a glass transition temperature (Tg) greater than or equal to 15°C and free of aromatic ring structures. Preferably, the high Tg monomers used according to the invention may have a glass transition temperature of 15°C to 200°C, more preferably 30°C to 170°C, more preferably 40°C to 160°C, and even more preferably 50°C to 100°C.

[0042] In principle, the high Tg monomers that can be used in this invention are not particularly limited, as long as they have the above-mentioned glass transition temperature and do not contain aromatic ring structures.

[0043] In embodiments of the invention, the high Tg monomer (c) may be selected from tert-butyl acrylate (tBA), trimethylcyclohexyl acrylate (TMCHA), cyclohexyl acrylate (CHA), N-vinylpyrrolidone (NVP), acrylamide (ACMO), dimethacrylamide (DMAA), diethylacrylamide (DEAA), and isobornyl acrylate (IBOA), preferably tert-butyl acrylate (tBA) and N-vinylpyrrolidone (NVP). When these monomers are included in the monomer composition forming the pressure-sensitive adhesive of the invention, the pressure-sensitive adhesive of the invention is able to maintain the adhesion and good appearance of the bonded product for a long time, while having good optical properties (in particular, low yellowing properties and good optical transparency).

[0044] Those skilled in the art will know that the glass transition temperature can be determined using dynamic scanning calorimetry (DSC). Specifically, 5 mg of untreated polymer sample is weighed into an aluminum crucible (25 μL volume) and sealed with a perforated lid. Measurements are performed using a Netzsch DSC 204F1. The procedure is conducted under nitrogen atmosphere. The sample is first cooled to -150 °C, then heated to +150 °C at a heating rate of 10 K / min, and then cooled again to -150 °C. The subsequent second heating curve is run again at 10 K / min, and the change in heat capacity is recorded. The glass transition is considered as a step in the temperature spectrum.

[0045] In addition to monomers a), b), and c), the monomer composition used to form the pressure-sensitive adhesive of the present invention may also include at least one monomer d) that is different from monomers a), b), and c) and is copolymerizable with monomers a), b), and c). Monomer d) used in the present invention is not particularly limited, as long as it is different from monomers a), b), and c) and is copolymerizable with monomers a), b), and c). Copolymerizable monomer d) is selected, for example, from: methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, sec-butyl methacrylate, amyl methacrylate, isoamyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, isooctyl methacrylate, etc. More advantageously, said monomer d) may be 2-ethylhexyl acrylate (2-EHA) or n-butyl acrylate (n-BA).

[0046] When a pressure-sensitive adhesive comprises a polymer formed by copolymerizing monomers a), b), and c) in particularly specific amounts as defined above, and optionally monomer d), the pressure-sensitive adhesive can simultaneously exhibit high refractive index, excellent optical stability, and outstanding dynamic shear strength, especially when the pressure-sensitive adhesive is formed into a sheet with a thickness of not less than 500 μm, particularly 1000 μm, and the sheet does not exhibit performance degradation even after prolonged use under humid and hot conditions. Therefore, the pressure-sensitive adhesive obtained from the monomer composition can maintain the adhesion and good appearance of the bonded product for a long time, while also exhibiting low yellowing properties and good optical transparency.

[0047] In a preferred embodiment of the present invention, the optically transparent pressure-sensitive adhesive of the present invention is obtained from a monomer composition comprising or consisting of the following:

[0048] a) 25 to 45% by weight, preferably 30 to 35% by weight, of monomers selected from 2-hydroxyethyl acrylate (2-HEA), 4-hydroxybutyl acrylate (4-HBA), and mixtures thereof;

[0049] b) 30 to 45% by weight, preferably 30 to 35% by weight, of monomers selected from benzyl acrylate (BZA), phenoxybenzyl acrylate (PBA), and mixtures thereof;

[0050] c) 5 to 40% by weight, preferably 5 to 30% by weight, of monomers selected from tert-butyl acrylate (tBA), N-vinylpyrrolidone (NVP), cyclohexyl acrylate (CHA), and mixtures thereof; and

[0051] d) Optionally, at least one monomer different from monomers a), b), and c) and capable of copolymerizing with monomers a), b), and c), preferably 2-ethylhexyl acrylate (2-EHA).

[0052] Based on the total weight of the monomer composition.

[0053] In a particularly preferred embodiment of the invention, the optically transparent pressure-sensitive adhesive of the invention is obtained from a monomer composition comprising or consisting of the following:

[0054] a) 25 to 45% by weight, preferably 30 to 35% by weight, of 4-hydroxybutyl acrylate (4-HBA);

[0055] b) 30 to 45% by weight, preferably 30 to 35% by weight, of benzyl acrylate (BZA) and / or phenoxybenzyl acrylate (PBA);

[0056] c) 5 to 40% by weight, preferably 5 to 30% by weight, of tert-butyl acrylate (tBA) and / or N-vinylpyrrolidone (NVP) and / or cyclohexyl acrylate (CHA); and

[0057] d) 2-Ethylhexyl acrylate (2-EHA),

[0058] Based on the total weight of the monomer composition.

[0059] In a further preferred embodiment of the present invention, the optically transparent pressure-sensitive adhesive of the present invention is obtained from a monomer composition comprising or consisting of the following:

[0060] a) 25 to 45% by weight, preferably 30 to 35% by weight, of 4-hydroxybutyl acrylate (4-HBA);

[0061] b) 30 to 45% by weight, preferably 30 to 35% by weight, of benzyl acrylate (BZA);

[0062] c) 5 to 40 wt%, preferably 5 to 30 wt%, of tert-butyl acrylate (tBA) and / or N-vinylpyrrolidone (NVP); and

[0063] d) 2-Ethylhexyl acrylate (2-EHA),

[0064] Based on the total weight of the monomer composition.

[0065] Methods for forming polymers from said monomers a), b), and c), and optionally monomer d), are known in the art. For example, the preparation of polymers can be accomplished by conventional free radical polymerization or controlled free radical polymerization, for example by copolymerizing monomers using conventional polymerization initiators and optionally chain transfer agents, by polymerization in bulk, in an emulsion such as in water or a liquid hydrocarbon, or in solution.

[0066] For example, the polymer is prepared by copolymerizing the monomers in a solvent, more particularly in a solvent having a boiling range of 50-150°C, preferably 60-120°C, using 0.01 to 5% by weight, especially 0.1 to 2% by weight (based on the total weight of the monomers in their respective cases) of a polymerization initiator.

[0067] All conventional initiators are suitable in principle. Examples of free radical sources are peroxides, hydroperoxides, and azo compounds, such as benzoyl peroxide, cyclohexanone peroxide, di-tert-butyl peroxide, cyclohexylsulfonylacetyl peroxide, diisopropyl percarbonate, tert-butyl peroctanoate, and benzyl benzoyl alcohol.

[0068] Preferred solvents for preparing the polymer are esters such as ethyl acetate, butyl acetate, especially ethyl acetate; hydrocarbons such as toluene; and mineral oils (solvent oils) particularly having a boiling range of 60 to 120°C; ketones, especially acetone, methyl ethyl ketone, methyl isobutyl ketone; and mixtures of the foregoing solvents. Particularly preferred solvents are mixtures containing isopropanol in an amount of 2 to 15% by weight, particularly 3 to 10% by weight, based on the solvent mixture used.

[0069] Preferably, the preparation (polymerization) of the polymer is followed by a concentration step, and further processing of the polymer is substantially solvent-free. The concentration of the polymer can be carried out in the absence of crosslinking agents and accelerators. However, it is also possible to add one of these compound classes to the polymer even before concentration, so that concentration is carried out in the presence of this / these substances.

[0070] After the concentration step, the polymer can be transferred to a mixing machine. Concentration and mixing can also optionally be carried out in the same reactor.

[0071] In a preferred embodiment, the polymer of the pressure-sensitive adhesive of the present invention is prepared in the form of a UV slurry by: mixing monomers a), b), and c), optionally monomer d), and a first initiator in the amounts according to the present invention to form a mixture; applying UV light radiation to initiate the prepolymerization of the mixture to form a first adhesive slurry; adding a second initiator, a crosslinking agent monomer, and a coupling agent such as a silane coupling agent to the first adhesive slurry to form a second adhesive slurry; and applying UV light radiation to initiate the polymerization of the second adhesive slurry, thereby forming the adhesive. Preferably, before applying UV light radiation to initiate the polymerization of the second adhesive slurry, the obtained second adhesive slurry is degassed under negative pressure and then coated onto a release film after degassed. There is no particular limitation that the first initiator and the second initiator used for UV-induced polymerization may be the same or different, and may be independently selected from the group consisting of: Irgacure 651, Irgacure 184, Omnirad 1173, Omnirad 819, Omnirad, TPO-L, Omnipol TP, or combinations thereof.

[0072] The crosslinking agent monomer is not particularly limited and may be selected from the following group: 1,6-hexanediol diacrylate, ethylene glycol dimethacrylate, or combinations thereof.

[0073] Therefore, in a particularly preferred embodiment, the optically transparent pressure-sensitive adhesive of the present invention may be a cross-linked pressure-sensitive adhesive.

[0074] According to a second aspect of the invention, an adhesive sheet is provided comprising an optically transparent pressure-sensitive adhesive according to a first aspect of the invention, said adhesive sheet having a thickness greater than or equal to 250 μm, preferably greater than 500 μm, more preferably 500 to 1000 μm.

[0075] In a preferred embodiment, the adhesive sheet of the present invention is provided in the form of a pressure-sensitive adhesive tape, preferably a transfer tape, i.e., a carrier-free pressure-sensitive adhesive tape with double-sided adhesive. In the transfer pressure-sensitive tape, the pressure-sensitive adhesive is applied between flexible pads before application, the flexible pads having a release layer and / or anti-sticking properties. During application, one pad can be removed first, the pressure-sensitive adhesive applied, and then the second pad removed. Therefore, the pressure-sensitive adhesive can be used directly to bond two surfaces. Using this carrier-free transfer pressure-sensitive tape with such pressure-sensitive adhesive allows for very precise bonding in terms of positioning and dosage.

[0076] The flexible backing is known to those skilled in the art and is also referred to as a release liner. In addition to covering the double-sided adhesive pressure-sensitive tape, the backing ensures that the pressure-sensitive adhesive is not smudged before application. However, such a backing is not an assembly of the pressure-sensitive tape, but merely an aid for its manufacture, storage, and / or further processing via stamping. Furthermore, unlike a permanent carrier, the backing is not firmly bonded to the adhesive layer but acts as a temporary carrier, i.e., a carrier removable from the pressure-sensitive adhesive layer.

[0077] According to a third aspect of the invention, the invention provides the use of the optically transparent pressure-sensitive adhesive or the adhesive sheet for bonding two substrates, preferably, one substrate being 3D curved and the other substrate being planar, single-curved or 3D curved.

[0078] In a preferred embodiment of the present invention, the substrate is a glass substrate or a polycarbonate substrate.

[0079] In a particularly preferred embodiment of the present invention, the substrate is a display screen, a smart car window, or architectural glass.

[0080] Example

[0081] General experimental instructions: Manufacturing of optically transparent pressure-sensitive adhesives

[0082] A mixture of acrylate monomers (monomer d), hydroxyl-containing acrylate monomers (monomer a), high refractive index monomers (monomer b), high Tg monomers (monomer c), and a first initiator (Igarcure 651, BASF (China)) was prepared according to the materials and amounts listed in the examples and comparative examples in Table 1. UV light radiation was applied to initiate the prepolymerization of the mixture (here, the UV radiation conditions are as follows: LED wavelength 365nm, intensity 0.5mW / cm). 2 Irradiation time 20 minutes) to prepolymerize it into a first adhesive slurry with a viscosity of 3000 cps.

[0083] A second adhesive slurry was obtained by adding a second initiator (Igarcure 184, BASF (China)), a crosslinking monomer (1,6-hexanediol diacrylate (HDDA), Sartoma Guangzhou Chemical Co., Ltd.), and a silane coupling agent (KBM403, Shin-Etsu Chemical Industry Co., Ltd.) to the first adhesive slurry. The obtained second adhesive slurry was then degassed under negative pressure and coated onto a polyethylene terephthalate (PET) release liner after degassed. UV light radiation (3000 mJ / cm²) was then applied. 2 This initiates the polymerization of the second adhesive slurry and ultimately transforms it into an adhesive layer.

[0084] Then, cover the exposed side of the adhesive layer with an additional PET release liner.

[0085] In the examples and comparative examples, the sum of the weights of monomers a), b), c), and d) is 100%, and the percentages of other components such as initiators and silane coupling agents are weight percentages relative to the sum of the weights of monomers a), b), c), and d).

[0086] In this way, adhesive sheets with thicknesses of 500 μm and 1000 μm were manufactured under their respective conditions.

[0087] The refractive index RI (at a wavelength of 589 nm), glass transition temperature Tg, and storage modulus G′ (at 25 °C) of the adhesives provided in each embodiment and comparative example were measured. Furthermore, the Δb value and yellowing index were measured after 1000 hours of storage at 105 °C, the haze value and transmittance after 1200 hours of storage at 85 °C and 85% relative humidity, the dynamic shear strength, the 90° peel adhesion to glass, and the results in the anti-rebound test were measured at layer thicknesses of 500 μm and 1000 μm, respectively. The corresponding measurement results are shown in Table 1.

[0088] Table 1

[0089]

[0090]

[0091] In Table 1: 2-Ethylhexyl acrylate (2-EHA) 4-HBA (4-Hydroxybutyl Acrylate) BZA benzyl acrylate PBA phenoxybenzyl acrylate tBA tert-butyl acrylate NVP N-vinylpyrrolidone

[0092] CHA cyclohexyl acrylate

[0093] Igarcure 651 First Initiator

[0094] Igarcure 184 First Initiator

[0095] Silane coupling agent

[0096] As shown in Table 1, the optically transparent pressure-sensitive adhesives of Examples 1 to 5, possessing the dynamic shear strength defined according to the present invention, achieved significantly better peel adhesion performance than the non-invention comparative examples at thicknesses of 500 μm and even 1000 μm, and all passed the anti-rebound test (i.e., no bubbles or delamination). This indicates that these pressure-sensitive adhesives can maintain good adhesion and appearance of the bonded products for a long time. The adhesives possess excellent mechanical properties (dynamic shear strength >1 MPa) and adhesive properties (peel force >20 N / cm), do not rebound or delamination, and also have good optical properties, particularly low yellowing properties (characterized by a low Δb value / yellowing index after aging) and good optical transparency. Furthermore, compared to the comparative examples, the optically transparent pressure-sensitive adhesives of Examples 1 to 5 according to the present invention did not exhibit whitening after aging at either thickness (characterized by a haze value less than or equal to 1), indicating that the optically transparent pressure-sensitive adhesives according to the present invention still possess advantageous optical stability at higher thicknesses. Therefore, the pressure-sensitive adhesive according to the present invention is particularly suitable for bonding substrates with 3D curved surfaces, such as bonding displays, automotive smart windows, or architectural glass.

[0097] Test methods

[0098] The tests performed on the embodiments and comparative examples according to the present invention and the corresponding information are summarized in Table 2 below, and will be described in detail below.

[0099] Table 2: Tests performed according to embodiments and comparative examples of the present invention

[0100]

[0101]

[0102] In Table 2, "RH" represents relative humidity.

[0103] I. Refractive Index Test

[0104] After placing the test sample at 23±1℃ and 50±5%RH for 24 hours, the refractive index RI value of the sample under a 589nm light source was measured using a Metricon prism coupler (model Metricon 2010 / M PRISM COUPLER).

[0105] II. Δb, Yellowing Index (YI) Test:

[0106] Accelerated aging test conditions: (105℃, 1000h)

[0107] Remove the release liner from one side of the obtained adhesive layer, then use a pressure roller to press the adhesive layer onto a 1 mm thick glass plate, ensuring there are no impurities or air bubbles at the bonding interface. Next, remove the release liner from the other side of the adhesive layer and place the resulting laminate in a vacuum bonding chamber with the adhesive side facing up. After covering with another 1 mm thick glass plate, vacuum bonding is performed under conditions below 200 Pa for 10 seconds. The resulting laminate is then degassed at 40°C, 0.5 MPa for 30 minutes. The resulting laminate is placed at 25°C and 50% relative humidity for 24 hours, then placed in an oven at 105°C. The Δb value is measured at 250 hours, 500 hours, and 1000 hours using a BYK spectro-guide (purchased from BYK-GARDNER GMBH, GERMANY), and the yellowing index (YI) is measured using a UV-Vis spectrophotometer. The BYK spectro-guide is used for calibration on a white slate through a blank glass slide.

[0108] III. Transmittance (T) and Haze Tests:

[0109] Accelerated aging test conditions: (85℃ / 85%RH, 1000h)

[0110] The laminate was manufactured using the method described above, but under the following conditions: the resulting laminate was placed at 25°C and 50% relative humidity for 24 hours, followed by experiments in a temperature and humidity chamber at 85°C and 85% relative humidity. Haze values ​​and transmittance were measured at 250, 500, and 1000 hours using a BYK haze-gard plus (purchased from BYK-GARDNER GMBH, GERMANY). Generally, excellent optical performance corresponds to a haze value less than 1 and a transmittance greater than 99%.

[0111] IV. 90° Peel Adhesion Test

[0112] Peel adhesion was measured at a temperature of 23℃±1℃ and a relative humidity of 50%±5%.

[0113] Test samples were cut to 20 mm widths and adhered to glass and polycarbonate (PC) substrates respectively. The substrates were cleaned and conditioned before measurement. The side facing away from the substrate was then lined with a 50 μm or other thickness plastic film, such as polyethylene terephthalate (PET) or polyvinyl chloride (PVC), to prevent stretching or deformation of the adhesive layer during testing. The test sample was then rolled onto the substrate using a 4 kg roller at a rolling rate of 10 m / min, five times. After the rolling treatment, the test sample was placed at 25°C and 50% relative humidity for 24 hours. The test sample was then inserted into a Zwick tensile testing machine at a 90° angle for peel force testing. Peel adhesion was measured at a speed of 300 mm / min. The test result for each sample is expressed as the average of three measurements (in N / cm).

[0114] V. Dynamic shear strength test:

[0115] Glass dimensions: 7cm*3cm*5mm

[0116] Sample size: 3cm*3cm

[0117] Test speed: 50mm / minute

[0118] Test temperature: 23℃

[0119] A piece of soda-lime glass measuring 7cm*3cm*5mm was plasma-treated, then a 3cm*3cm adhesive tape sample was bonded to it. Another piece of plasma-treated glass was then vacuum-bonded. After bonding, the sample was degassed at 80℃ and 0.5MPa for 45 minutes. After degassed, the sample was left at room temperature for 24 hours, and then... Figure 1 As shown, the two pieces of glass were pulled apart at a 180° angle at a speed of 50 mm / min to determine the maximum force required for this, and the result was expressed in N / cm. 2 count.

[0120] VI. Anti-rebound test:

[0121] like Figure 2 As shown, optically transparent pressure-sensitive tape was applied to a 19cm x 4cm, 0.6mm thick glass sheet. The release liner on the other side was removed, and the tape was then applied to a 23cm x 4.5cm, 0.4mm thick glass sheet. The sheet was then mounted on a fixture with a curvature of 800mm, and both sides of the glass were secured. The adhesion of the tape to the glass was tested while the sheet was bent.

[0122] Test conditions: 105℃, 1000h. Observe the adhesion effect of the tape on the curved substrate under high temperature and high humidity conditions. If there are no bubbles on the tape and no peeling or delamination caused by rebound at the edge of the adhesive, the result is recorded as "+", otherwise it is recorded as "-".

Claims

1. An optically transparent pressure-sensitive adhesive, wherein the optically transparent pressure-sensitive adhesive has a dynamic shear strength of at least 1 MPa, preferably 1 MPa to 3 MPa, as measured according to ASTM D1002, and has a Δb value of less than 2 as measured according to EN ISO 11664, or a yellowing index of less than or equal to 1.5 as measured according to ASTM E313, after being stored at 105°C for 1000 hours.

2. The optically transparent pressure-sensitive adhesive according to claim 1, wherein when the optically transparent pressure-sensitive adhesive is formed into a sheet with a thickness of not less than 500 μm, particularly 1000 μm, it still has a dynamic shear strength of at least 1 MPa, preferably 1 MPa to 3 MPa, as measured according to ASTM D1002, and has a Δb value of less than 2 as measured according to EN ISO 11664, or a yellowing index of less than or equal to 1.5 as measured according to ASTM E313, after being stored at 105°C for 1000 hours.

3. The optically transparent pressure-sensitive adhesive according to claim 1 or 2, wherein the optically transparent pressure-sensitive adhesive has a haze value of less than or equal to 1, preferably less than or equal to 0.5, more preferably less than or equal to 0.2, even more preferably less than or equal to 0.1, and particularly 0, as measured according to ASTM D1003 after being stored at 85°C and 85% relative humidity for 1200 hours.

4. The optically transparent pressure-sensitive adhesive according to any one of claims 1 to 3, wherein the optically transparent pressure-sensitive adhesive has a refractive index greater than or equal to 1.510 at a wavelength of 589 nm.

5. The optically transparent pressure-sensitive adhesive according to any one of claims 1 to 4, wherein the optically transparent pressure-sensitive adhesive is derived from a monomer composition comprising: a) at least one hydroxyl-containing acrylate monomer, at least 25% by weight, preferably 25 to 45% by weight, more preferably 30 to 35% by weight; b) At least one high-refractive-index monomer, at least 30% by weight, preferably 30 to 45% by weight, more preferably 35 to 40% by weight or 30 to 35% by weight; c) at least one high-Tg monomer comprising 5% or more by weight, preferably 8 to 40% by weight, more preferably 10 to 30% by weight; and d) Optionally, at least one monomer that is different from monomers a), b) and c) and can copolymerize with monomers a), b) and c), based on the total weight of the monomer composition.

6. The optically transparent pressure-sensitive adhesive according to claim 5, wherein monomer a) is a (meth)acrylate hydroxy C1-C6 alkyl ester, preferably selected from 4-hydroxybutyl acrylate (4-HBA) and 2-hydroxyethyl acrylate (2-HEA), more preferably 4-hydroxybutyl acrylate (4-HBA).

7. The optically transparent pressure-sensitive adhesive according to claim 5, wherein monomer b) is a monomer having a refractive index greater than or equal to 1.510, preferably 1.510 to 1.580, more preferably 1.515 to 1.580 and containing at least one aromatic ring, preferably said monomer b) is a (meth)acrylate containing at least one aromatic ring, and preferably selected from benzyl acrylate (BZA), phenoxybenzyl acrylate (PBA), 2-phenoxyethyl acrylate (2-PEA) and o-phenylphenoxyethyl acrylate (OPPEA), preferably benzyl acrylate (BZA) and phenoxybenzyl acrylate (PBA).

8. The optically transparent pressure-sensitive adhesive according to claim 5, wherein monomer c) is a monomer without an aromatic ring structure having a glass transition temperature Tg greater than or equal to 15°C, preferably 15°C to 200°C, preferably 30°C to 200°C, more preferably 50°C to 200°C, and even more preferably 50°C to 180°C, and preferably selected from tert-butyl acrylate (tBA), trimethylcyclohexyl acrylate (TMCHA), cyclohexyl acrylate (CHA), N-vinylpyrrolidone (NVP), acrylamide (ACMO), dimethacrylamide (DMAA), diethylacrylamide (DEAA), and isobornyl acrylate (IBOA), and even more preferably tert-butyl acrylate (tBA), N-vinylpyrrolidone (NVP), and cyclohexyl acrylate (CHA).

9. An adhesive sheet comprising an optically transparent pressure-sensitive adhesive according to any one of claims 1 to 8, wherein the adhesive sheet has a thickness of 250 μm or more, preferably 500 μm or more, and more preferably 500 to 1000 μm.

10. The use of the optically transparent pressure-sensitive adhesive according to any one of claims 1 to 8 or the adhesive sheet according to claim 9 for bonding two substrates, preferably, one substrate is 3D curved and the other substrate is planar, single-curved or 3D curved.

11. The use according to claim 10, wherein the substrate is a glass substrate or polycarbonate.

12. The use according to claim 10 or 11, wherein the substrate is a display screen, a smart automotive window, or architectural glass.

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