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

Abstract

The present invention relates to an optically clear pressure-sensitive adhesive, which has a dynamic shear strength of at least 1 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 h. Moreover, the present invention further relates to an adhesive sheet comprising the optically clear pressure-sensitive adhesive, and use of the optically clear pressure-sensitive adhesive or the adhesive sheet for bonding two substrates.

Description

Optically clear pressure-sensitive adhesive, adhesive sheet and use thereofTECHNICAL FIELD

[0001] The present invention relates to an optically clear pressure-sensitive adhesive, an adhesive sheet comprising the optically clear pressure-sensitive adhesive, and use of the optically clear pressure-sensitive adhesive or the adhesive sheet for bonding two substrates.BACKGROUND

[0002] The joining of separate elements is one of the central processes in manufacturing technology. In addition to other methods, such as welding and soldering, adhesive bonding, i.e., joining of elements using an adhesive, in particular adhesives in sheet form such as adhesive tapes, is an important method nowadays for joining separate elements. Pressure-sensitive adhesives are known for use in manufacturing and everyday life. They are permanently tacky and glueable under conventional environmental conditions, and can be easily applied to a substrate with the aid of pressure and remain adhered thereto, but can later be removed from the substrate more or less without leaving any residue. Due to their nature, these adhesives are widely used in various mechanical, electronic, automotive and construction areas.

[0003] The diversified development of products has also gradually led to the emergence of many application scenarios for curved surface bonding, especially for bonding in the fields of electronics, automobiles and construction, such as for bonding of display screens, smart car windows for automobile, or architectural glasses. This is because, for example, the glasses used in display screens, smart car windows for automobile, or buildings with a specific design are usually not completely planar, but often have a 3D curved surface, which makes the requirements on the adhesives more stringent. On the one hand, for fixing this type of glass to a corresponding substrate by adhesion, it is required that the adhesive has a sufficient thickness so as to be able to compensate for voids caused by irregular adhesive surfaces, easily follow and adhere to the curved surfaces, and at the same time, it is also required that the adhesive has a low yellowing property (typically, a Ab value of less than 2 or a yellowing index of less than or equal to 1.5),especially to overcome the occurrence of yellowing when the thickness of the adhesive increases. On the other hand, the adhesive is required to have good optical properties that are required for conventional optically clear pressure sensitive adhesives, such as a high optical transparency and a low haze value (preventing the adhesive from whitening and clouding). In addition, what is more important in such application scenarios is that it is also necessary to possess superior mechanical properties and adhesive properties, so that when bonding 3D curved substrates, the stress caused by bending can be absorbed and thereby preventing rebound and de-bonding at the edge bonding position of the curved surfaces. That is, no defects and delamination occur after bonding, and a good bonding state is maintained even under conditions of high temperature and high humidity.

[0004] When compensating for voids caused by irregular adhesive surfaces, it is often necessary to provide the pressure-sensitive adhesive in a larger thickness. Under such circumstances, the adhesive provided in a greater thickness is affected by the application environment, such as hot and humid weather outdoors. Over time, this can cause the bonded products to no longer possess a satisfactory appearance (e.g., the generation of bubbles, the occurrence of yellowing and whitening, etc.), and sometimes even separation of the bonded products.

[0005] Therefore, there is still a need for the following adhesives: the adhesives are able to maintain the good adhesive effect and appearance of the products for a long time in products that are bonded to a curved substrate, the adhesives possess more excellent mechanical and adhesive properties, no rebound and no delamination, and at the same time, they also have good optical properties, in particular low yellowing properties and good durability when made into a high thickness.SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide an optically clear pressure-sensitive adhesive that is capable of maintaining good bonding effects and appearance of the products over a long period of time in bonded products, and at the same time, possesses good opticalproperties per se (in particular, low yellowing properties and good optical transparency), as well as more excellent mechanical properties and adhesion properties.

[0007] In addition, it is also an object of the present invention to provide an adhesive sheet comprising the optically clear pressure-sensitive adhesive, which is particularly useful for bonding substrates with a 3D curved surface. On the one hand, the sheet can have a thickness of not less than 250 pm, such as not less than 500 pm, or even 1000 pm, so that voids caused by irregular 3D curves can be fully compensated. On the other hand, the sheet can simultaneously exhibit both low yellowing properties and good optical transparency at a higher thickness. The sheet can maintain good adhesion and good appearance of the products for a long time even after being used for a long time under hot and humid conditions.

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

[0009] Accordingly, in a first aspect, the present invention provides an optically clear pressuresensitive adhesive, wherein the optically clear pressure-sensitive adhesive has a dynamic shear strength of at least 1 MPa, preferably of 1 MPa to 3 MPa as measured according to ASTM D1002; and a Ab 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 h. In particular, the optically clear pressure-sensitive adhesive, when formed into a sheet with a thickness of not less than 500 pm, in particular 1000 pm, still has a dynamic shear strength of at least 1 MPa, preferably of 1 MPa to 3 MPa as measured according to ASTM D1002; and a Ab value of less than 2, or a yellowing index of less than or equal to 1.5, after being stored at 105 °C for 1000 h.

[0010] In a second aspect, the invention provides an adhesive sheet comprising the optically clear pressure-sensitive adhesive according to the first aspect of the invention, wherein theadhesive sheet has a thickness greater than or equal to 250 pm, preferably greater than 500 pm, more preferably 500 to 1000 pm. The sheet can still exhibit low yellowing properties and good optical transparency at a higher thickness. After being used for a long time under hot and humid conditions, good adhesion and good appearance of the products can also be maintained for a long time.

[0011] In a third aspect, the invention provides use of the optically clear pressure-sensitive adhesive according to the first aspect of the invention or the adhesive sheet according to the second aspect of the invention for bonding two substrates, preferably wherein one of the substrates is 3D curved, and the other one is flat, single curved or 3D curved.BRIEF DESCRIPTION OF FIGURES

[0012] The above and other aspects, features and advantages of certain embodiments of the present invention will be more apparent from the following detailed description taken in conjunction with the attached figures, in which:

[0013] Fig. 1 shows a schematic diagram of dynamic shear strength test; and

[0014] Fig. 2 shows a schematic diagram of the rebound resistance test.DETAILED DESCRIPTION

[0015] The above objects are achieved by the subject matters as defined according to the invention. Preferred designs according to the invention result from further developments according to the invention and the following embodiments.

[0016] The embodiments referred to below as preferred can be combined in particularly preferred embodiments with features of further embodiments referred to as preferred. Therefore, very particular preference is given to combinations of two or more of the embodiments referred to below as particularly preferred. It is also preferred that features of one embodiment, which is to a certain extent referred to as being preferred, are combined with one or more other features of another embodiment, which is to a certain extent referred to as beingpreferred. Therefore, combinations of individual features with one another are encompassed in the invention, and under such circumstances also different levels of preference. Therefore, for example, a combination of a first feature referred to as "preferred" with a second feature referred to as "particularly preferred" is encompassed in the present invention. The characteristics of preferred adhesive sheets and uses here derive from the characteristics of preferred pressure-sensitive adhesives. The characteristics of preferred pressure-sensitive adhesives and uses also derive from the characteristics of preferred adhesive sheets.

[0017] As is well known, pressure-sensitive adhesives are understood according to the present invention to mean substances that are particularly permanently tacky and adhesive, especially at room temperature. A pressure-sensitive adhesive is characterized in that it can be applied to a substrate by pressure and remains adhered to the substrate, without the need to define in detail the pressure to be applied and the duration of action of this pressure. In some cases, depending on the exact natures of the pressure-sensitive adhesive, temperature, air humidity and the substrate, the action of a minimum pressure of gentle contact for a short period of time is sufficient to achieve the adhesion effect. In other cases, a higher pressure and a longer action time can also be necessary.

[0018] Pressure-sensitive adhesives have specific characteristic viscoelastic properties, which result in long-lasting adhesive properties. Their characteristics are that when they are mechanically deformed, there is both a viscous flow process and the formation of elastic restoring forces. The two processes are in a specific relationship to each other with respect to their respective proportions, which depends not only on the precise composition, structure and degree of cross-linking of the pressure-sensitive adhesive substance, but also on the rate and duration of deformation, as well as on the temperature.

[0019] A certain proportion of viscous flow is necessary to achieve adhesion. The viscous component generated only by macromolecules with relatively large mobility allows efficient wetting of the substrates to be bonded and efficient flow onto them. A high viscous flowcomponent leads to a high pressure-sensitive adhesive property (also called as surface tack), and thus often also to a high adhesion. Due to the lack of flowable components, generally speaking, highly cross-linked systems, crystalline or glassy cured polymers have only little or no pressuresensitive adhesiveness at all.

[0020] A certain proportion of elastic restoring force is necessary to achieve cohesion. They result, for example, from macromolecules whose chains are very long and highly curled, and those that are physically or chemically cross-linked, and allow the transmission of forces acting on the adhesive bond. They result in the adhesive bond being able to sufficiently withstand longterm loads acting on it over a relatively long period of time, for example in the form of long-term shear loads.

[0021] In order to more accurately describe and quantify the measures of the elastic and viscous components, and the relationship between the components, the variables that can be determined by means of dynamic mechanical analysis (DMA, in accordance with DIN EN ISO 6721) can be used: storage modulus (G') and loss modulus (G"). G' is a measure of the elastic component of a substance, and G" is a measure of the viscous component of the substance. Both variables depend on deformation frequency and temperature.

[0022] The variables can be determined with the aid of a rheometer. Here, for example, the material to be tested is exposed to a sinusoidally 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 offset of this deformation is measured relative to the introduction of shear stress. This time offset is called the phase angle 6.

[0023] The storage modulus G' is defined as follows: G'= (r / y) »cos(6) (r - shear stress, y - deformation, 6 - phase angle - phase shift between the shear stress vector and the deformation vector). The loss modulus G" is defined as follows: G" - (r / y) «sin(5) (r - shear stress, y -deformation, 6 - phase angle - phase shift between the shear stress vector and the deformation vector).

[0024] If at room temperature, defined here as 23 °C, in the deformation frequency range of 10°- 101rad / sec, G' lies at least partially in the range of 103-107Pa, and if G" lies also at least partially within this range, the substance is generally considered to be pressure-sensitive adhesive, and is defined as pressure-sensitive adhesive in the sense of the present invention. "Partially" means that at least a portion (at least a segment) of the G' curve lies within a window spanned by the deformation frequency range (abscissa) from 10° rad / s (end points included) to 101rad / s (end points included), and the range (ordinate) of G’ values from 103Pa (end points included) to 107Pa (end points included). For G", this applies accordingly.

[0025] The optically clear pressure-sensitive adhesive according to the first aspect of the invention has a dynamic shear strength of at least 1 MPa, preferably of 1 MPa to 3 MPa as measured according to ASTM D1002; and a Ab 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 h.

[0026] The Ab value refers to the change in the color of the sample on the yellow-blue axis (b- axis) in color measurements. It is usually used to indicate the degree of yellowing of a material or product under the influence of aging, light or other environmental factors. The larger Ab, the more obvious the degree of yellowing. The thickness of the optically clear adhesive will affect its yellowing Ab value. Generally speaking, as the thickness of the adhesive increases, the yellowing Ab value also increases. This is because a thicker adhesive layer will absorb and scatter more light, resulting in a more pronounced color change. Therefore, in order to ensure that the optical properties of the final product meet the requirements, the effect of thickness on yellowing needs to be considered. In a preferred embodiment of the present invention, when the optically clear pressure-sensitive adhesive is made into a sheet with a thickness of not less than 500 pm, especially 1000 pm, it still has a dynamic shear strength of at least 1 MPa, preferably of 1 MPa to3 MPa as measured according to ASTM D1002; and a Ab 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 h.

[0027] In embodiments of the invention, the dynamic shear strength can 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 clear pressure-sensitive adhesive is made into a sheet with a thickness of 500 pm, the dynamic shear strength can be at least 1.3 MPa, such as greater than 2 MPa, 1.3 MPa to 1.7 MPa, or 1.5 MPa. For example, when the optically clear pressure-sensitive adhesive is made into a sheet with a thickness of 1000 pm, the dynamic shear strength can be at least 1 MPa, such as 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 adhesion and good appearance of the products in the bonded products for a long time.

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

[0029] In preferred embodiments of the present invention, the optically clear pressure-sensitive adhesive has a haze value of less than or equal to 1, preferably of less than or equal to 0.5, more preferably of less than or equal to 0.2, further preferably of less than or equal to 0.1, and particularly of 0, as measured according to ASTM D1003, after being stored at 85 °C and 85% relative humidity for 1200 h. For example, when the optically clear pressure-sensitive adhesive is made into a sheet with a thickness of 500 pm, the haze value can be less than or equal to 0.4, less than or equal to 0.3, or less than or equal to 0.1, such as 0 to 0.3, or 0 to 0.1. For example, when the optically clear pressure-sensitive adhesive is made into a sheet with a thickness of 1000 pm, the haze value can 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, such as 0 to 0.6, or 0 to 0.2.

[0030] In preferred embodiments of the present invention, the optically clear pressure-sensitive adhesive has a refractive index of greater than or equal to 1.510, such as 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 clear pressure-sensitive adhesive can be derived from a monomer composition comprising: a) greater than or equal to 25 wt%, preferably 25 to 45 wt%, and more preferably 30 to 35 wt% of at least one hydroxyl-containing acrylate monomer; b) greater than or equal to 30 wt%, preferably 30 to 45 wt%, and more preferably 35 to 40 wt% or 30 to 35 wt% of at least one monomer with a high refractive index;c) greater than or equal to 5 wt%, preferably 8 to 40 wt%, and more preferably 10 to 30 wt% of at least one high Tg monomer; and d) optionally at least one monomer, which is different from the monomers a), b) and c), and is copolymerizable with the monomers a), b) and c), based on the total weight of the monomer composition.

[0032] In embodiments according to the present invention, the hydroxyl-containing acrylate monomer a) can be a hydroxy Ci-Ce alkyl (meth) acrylate, preferably selected from the group consisting of 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 3-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, and 6-hydroxyhexyl (meth) acrylate, further preferably selected from the group consisting of 4-hydroxybutyl acrylate (4-HBA) and 2- hydroxyethyl acrylate (2-HEA), more preferably 4-hydroxybutyl acrylate (4-HBA).

[0033] Within the scope of the present invention, the expression "(meth) acrylic acid" is intended to mean acrylic acid and / or methacrylic acid. Therefore, hydroxy Ci-Ce alkyl (meth) acrylate correspondingly encompasses hydroxy Ci-Ce alkyl acrylate and / or hydroxy Ci-Ce alkyl methacrylate.

[0034] In embodiments according to the invention, a monomer with a high refractive index is understood to mean a monomer having a refractive index greater than or equal to 1.510 at a wavelength of 589 nm and in particular containing at least one aromatic ring structure. Preferably, the monomer with a high refractive index that can be used according to the present invention can have a refractive index of 1.510 to 1.580, preferably 1.515 to 1.580.

[0035] In recent years, with the diversification of optical elements, refractive index has become an important property. As examples of applications using adhesives with a high refractive index, the following can be listed: hybrid aspherical lenses composed of composites of glass and resin, dichroic lenses, prisms, architectural glass, and smart windows of automobiles, etc. Adhesives for these applications require a high refractive index, and in principle, the monomer with a highrefractive index that can be used in the present invention is not particularly limited, as long as it has the above-mentioned refractive indexes and in particular contains at least one aromatic ring structure. By using monomers containing these aromatic ring structures in the monomer composition of the present invention, the refractive index of the resulting adhesive layer can be increased.

[0036] In embodiments according to the present invention, the monomer b) with a high refractive index can be a (meth) acrylate comprising at least one aromatic ring, preferably selected from the group consisting of benzyl acrylate (BZA), phenoxybenzyl acrylate (PBA), 2- phenoxyethyl acrylate (2-PEA) and o-phenylphenoxyethyl acrylate (OPPEA), preferably benzyl acrylate (BZA) and phenoxybenzyl acrylate (PBA). When these monomers are included in the monomer composition forming the pressure-sensitive adhesive of the present invention, the pressure-sensitive adhesive of the present invention advantageously has particularly excellent optical properties, especially a high refractive index greater than or equal to 1.510 at a wavelength of 589 nm.

[0037] In embodiments according to the invention, a high Tg monomer is understood to mean a monomer with a glass transition temperature (Tg) greater than or equal to 15 °C and which does not contain an aromatic ring structure. Preferably, the high Tg monomer that can be used according to the present invention can have a glass transition temperature of 15 °C to 200 °C, preferably 30 °C to 170 °C, more preferably 40 °C to 160 °C, further preferably 50 °C to 100 °C.

[0038] In principle, the high Tg monomer that can be used in the present invention is not particularly limited, as long as it has the above-mentioned glass transition temperature and does not contain an aromatic ring structure.

[0039] In embodiments according to the present invention, the high Tg monomer c) can be selected from the group consisting of tert-butyl acrylate (tBA), trimethylcyclohexyl acrylate (TMCHA), cyclohexyl acrylate (CHA), N-vinylpyrrolidone (NVP), acryloylmorpholine (ACMO),dimethylacrylamide (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 present invention, the pressure-sensitive adhesive of the present invention can maintain adhesion and good appearance of the products in the bonded products for a long time, and the same time, have good optical properties (especially, low yellowing properties and good optical transparency).

[0040] As is known to those skilled in the art, the glass transition temperature can be determined by means of dynamic scanning calorimetry (DSC). Specifically, 5 mg of untreated polymer sample is weighed into an aluminum crucible (volume 25 pL), and the crucible is closed with a perforated lid. Measurements are performed using a DSC 204 Fl from Netzsch. Operations are performed under nitrogen. The sample is first cooled to -150 °C, then heated to + 150 °C at a heating rate of 10 K / min, and cooled again to -150 °C. A 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 thermogram.

[0041] In addition to the monomers a), b) and c), the monomer composition used to form the pressure-sensitive adhesive of the present invention can also comprise at least one monomer d) that is different from the monomers a), b) and c) and is copolymerizable with the monomers a), b) and c). The monomer d) that can be used in the present invention is not particularly limited, as long as it is different from the monomers a), b) and c) and is copolymerizable with the monomers a), b) and c). The copolymerizable monomer d) is for example selected from the group consisting of: methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, sec-butyl (meth) acrylate, amyl (meth) acrylate, isoamyl (meth) acrylate, hexyl (meth) acrylate, heptyl (meth) acrylate, octyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, iso-octyl (meth) acrylate and the like. More advantageously, the monomer d) can be 2-ethylhexyl acrylate (2-EHA) or n-butyl acrylate (n-BA).

[0042] When the pressure-sensitive adhesive comprises a polymer formed by copolymerization of the monomers a), b) and c) and optionally monomer d) as defined above, in particular in specific amounts, the pressure-sensitive adhesive is able to simultaneously exhibit a high refractive index and an excellent optical stability, as well as an excellent dynamic shear strength, especially when the pressure-sensitive adhesive is made into a sheet with a thickness of not less than 500 pm, especially 1000 pm. Moreover, the sheet does not deteriorate in performance even after being used under humid and hot conditions for a long time. Therefore, the pressuresensitive adhesive obtained from the monomer composition can maintain adhesion and good appearance of the products in the bonded products for a long time, and the same time, have low yellowing properties and good optical transparency.

[0043] In preferred embodiments according to the invention, the optically clear pressuresensitive adhesive of the invention is derived from a monomer composition comprising or consisting of: a) 25 to 45 wt%, preferably 30 to 35 wt% of monomers selected from the group consisting of 2- hydroxyethyl acrylate (2-HEA), 4-hydroxybutyl acrylate (4-HBA) and mixtures thereof; b) 30 to 45 wt%, preferably 30 to 35 wt% of monomers selected from benzyl acrylate (BZA), phenoxybenzyl acrylate (PBA) and mixtures thereof; c) 5 to 40 wt%, preferably 5 to 30 wt% of monomers selected from t-butyl acrylate (tBA), N-vinyl pyrrolidone (NVP), cyclohexyl acrylate (CHA) and mixtures thereof; and d) optionally at least one monomer, which is different from the monomers a), b) and c), and is copolymerizable with the monomers a), b) and c), preferably 2-ethylhexyl acrylate (2-EHA), based on the total weight of the monomer composition.

[0044] In particularly preferred embodiments according to the invention, the optically clear pressure-sensitive adhesive of the invention is derived from a monomer composition comprising or consisting of: a) 25 to 45 wt%, preferably 30 to 35 wt% of 4-hydroxybutyl acrylate (4-HBA);b) 30 to 45 wt%, preferably 30 to 35 wt% of benzyl acrylate (BZA) and / or phenoxybenzyl acrylate (PBA); c) 5 to 40 wt%, preferably 5 to 30 wt% of tert-butyl acrylate (tBA) and / or N-vinyl pyrrolidone (N VP) and / or cyclohexyl acrylate (CHA); and d) 2-ethylhexyl acrylate (2-EHA), based on the total weight of the monomer composition.

[0045] In further preferred embodiments according to the invention, the optically clear pressuresensitive adhesive of the invention is derived from a monomer composition comprising or consisting of: a) 25 to 45 wt%, preferably 30 to 35 wt% of 4-hydroxybutyl acrylate (4-HBA); b) 30 to 45 wt%, preferably 30 to 35 wt% of benzyl acrylate (BZA); c) 5 to 40 wt%, preferably 5 to 30 wt% of tert-butyl acrylate (tBA) and / or N-vinyl pyrrolidone (NVP); and d) 2-ethylhexyl acrylate (2-EHA), based on the total weight of the monomer composition.

[0046] Methods for forming polymers from the monomers a), b) and c) and optionally monomer d) are known in the art. For example, the preparation of the polymers can be accomplished by conventional free-radical polymerization or controlled free-radical polymerization, for example by the following manners: copolymerizing the monomers using customary polymerization initiators and optionally chain transfer agents, by polymerization in bulk, in emulsion for example in water or liquid hydrocarbons, or in solution.

[0047] For example, the polymers are 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 wt%, especially 0.1 to 2 wt%, based in each case on the total weight of the monomers, of a polymerization initiator.

[0048] All customary initiators are suitable in principle. Examples of free radical sources are peroxides, hydroperoxides and azo compounds, such as dibenzoyl peroxide, cyclohexanone peroxide, di-tert-butyl peroxide, cyclohexylsulfonylacetyl peroxide, di-isopropyl percarbonate, tert-butyl peroctoate and benzopinacol.

[0049] Preferred solvents for preparing the polymers are esters such as ethyl acetate, butyl acetate, especially ethyl acetate; hydrocarbons such as toluene, and in particular mineral spirits (solvent oils) having a boiling range of 60 to 120°C; ketones, especially acetone, methyl ethyl ketone, methyl isobutyl ketone; and mixtures of the aforementioned solvents. Particularly preferred solvents are mixtures comprising isopropanol in an amount of 2 to 15 wt%, in particular 3 to 10 wt%, based on the solvent mixture used.

[0050] Preferably, the preparation (polymerization) of the polymer is followed by a concentration step, and further processing of the polymer is essentially solvent-free. Concentration of the polymer can be carried out in the absence of crosslinkers and accelerator substances. However, it is also possible to add one of these compound classes to the polymer even before concentration, so that the concentration is then carried out in the presence of this substance / these substances.

[0051] After the concentration step, the polymer can be transferred to a compounder. Concentration and mixing can optionally also be carried out in the same reactor.

[0052] In preferred embodiments, the polymer of the pressure-sensitive adhesive of the present invention is prepared in the form of a UV slurry by: mixing the monomers a), b) and c), optionally monomer d) and a first initiator to form a mixture; applying UV light radiation to initiate prepolymerization of the mixture to form a first adhesive slurry; adding a second initiator, a crosslinker 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 polymerization of the second adhesive slurry, thereby forming the adhesive. Preferably, before applying UV lightradiation to initiate polymerization of the second adhesive slurry, the obtained second adhesive slurry is de-bubbled (degassed) under negative pressure conditions, and is coated onto a release film after de-bubbling. The first initiator and the second initiator used for UV light radiation- initiated polymerization are the same or different, and are not particularly limited, and can be independently selected from the following: Irgacure 651, Irgacure 184, Omnirad 1173, Omnirad 819, Omnirad, TPO-L, Omnipol TP, or combinations thereof.

[0053] The crosslinker monomers are not particularly limited and can be selected from the following: 1,6-hexanediol diacrylate, ethylene glycol dimethacrylate, or combinations thereof.

[0054] Therefore, in particularly preferred embodiments, the optically clear pressure-sensitive adhesive of the present invention can be a cross-linked pressure-sensitive adhesive.

[0055] According to a second aspect of the invention, there is provided an adhesive sheet comprising the optically clear pressure-sensitive adhesive according to the first aspect of the invention, the adhesive sheet has a thickness greater than or equal to 250 pm, preferably greater than 500 pm, more preferably 500 to 1000 pm.

[0056] In preferred embodiments, the adhesive sheet of the invention is provided in the form of a pressure-sensitive adhesive tape, preferably a transfer adhesive tape, i.e., a double-sided adhesive carrier-free pressure-sensitive adhesive tape. In transfer pressure-sensitive adhesive tapes, the pressure-sensitive adhesive is applied between flexible liners prior to application, the flexible liners are provided with a release layer and / or possess anti-tack properties. In applications, one liner can be removed first, the pressure-sensitive adhesive is applied, and then the second liner is removed. Therefore, pressure-sensitive adhesives can be used directly to bond two surfaces. With this pressure-sensitive adhesive carrier-free transfer pressure-sensitive adhesive tape, very precise bonding in terms of positioning and dosage can be achieved.

[0057] The flexible liners are known to those skilled in the art, and are also known as release liners. In addition to covering the double-sided adhesive pressure-sensitive adhesive tape, the liners ensure that the pressure-sensitive adhesive does not stain prior to application. However, such liners are not components of pressure-sensitive adhesive tapes, but are merely aids fortheir manufacture, storage and / or further processing by stamping. Furthermore, unlike a permanent carrier, the liner is not firmly bonded to the adhesive layer, but acts as a temporary carrier, i.e., a carrier that is removable from the pressure-sensitive adhesive layer.

[0058] According to a third aspect of the present invention, the present invention provides use of the optically clear pressure-sensitive adhesive or the adhesive sheet for bonding two substrates, preferably wherein one of the substrates is 3D curved, and the other one is flat, single curved or 3D curved.

[0059] In preferred embodiments according to the present invention, the substrates are glass substrates or polycarbonates.

[0060] In particularly preferred embodiments according to the present invention, the substrates are display screens, smart car windows for automobile, or architectural glasses.Examples

[0061] General description of the experiments: manufacture of optically clear pressure-sensitive adhesives

[0062] A mixture of an acrylate monomer (monomer d)), a hydroxyl-containing acrylate monomer (monomer a)), a monomer with a high refractive index (monomer b)), a monomer with a high Tg (monomer c)), and a first initiator (Igarcure 651, BASF (China)) was prepared according to the materials and amounts in each of the Examples and Comparative Examples listed in Table 1. A UV light radiation was applied to initiate pre-polymerization of the mixture (here, conditions for the UV radiation were as follows: LED, with a wavelength of 365 nm, an intensity of 0.5mW / cm2, and an irradiation time of 20 minutes), so that the mixture was pre-polymerized into a first adhesive slurry with a viscosity of 3,000 cps.

[0063] To the first adhesive slurry, a second initiator (Igarcure 184, BASF (China)), a crosslinker monomer (1,6-hexanediol diacrylate (HDDA), Sartomer Guangzhou Chemicals Limited), and a silane coupling agent (KBM403, Shin-Etsu Chemical Co., Ltd.) were added to obtain a second adhesive slurry. Then, the obtained second adhesive slurry was de-bubbled under a negative pressure, and after de-bubbling, coated on a polyethylene terephthalate (PET) release liner. An UV light radiation (3,000 mJ / cm2) was applied to initiate polymerization of the second adhesive slurry and ultimately converted it into an adhesive layer.

[0064] Afterwards, an additional PET release liner was coated on the exposed side of the adhesive layer.

[0065] In the examples and comparative examples, the sum of the weight percentages of the monomer a), monomer b), monomer c) and monomer d) was taken as 100%, and the percentages of other components such as initiators and silane coupling agents were weight percentages relative to the sum of the weight percentages of the monomer a), monomer b), monomer c) and monomer d).

[0066] In this way, adhesive sheets with thicknesses of 500 pm and 1000 pm were manufactured in each case.

[0067] The adhesives provided in each of the examples and comparative examples were measured for their refractive index Rl (at a wavelength of 589 nm), glass transition temperature Tg and storage modulus G' (at 25 °C). The Ab values and the yellowing indexes after being stored at 105 °C for 1000 h, the haze values, the transmittances, and the dynamic shear strengths after being stored at 85 °C and a relative humidity of 85% for 1200 h, the 90° peel adhesion to glass,and the results in a rebound resistance test were measured at layer thicknesses of 500 pm and1000 pm, respectively. The corresponding measurement results are shown in Table 1.Table 1In Table 1:2-EHA 2-ethylhexyl acrylate4-HBA 4-hydroxybutyl acrylateBZA benzyl acrylatePBA phenoxybenzyl acrylate tBA tert-butyl acrylateNVP N-vinylpyrrolidoneCHA cyclohexyl acrylateIgarcure 651 a first initiatorIgarcure 184 a first initiatorSilane a silane coupling agent

[0068] As can be seen from the results in Table 1, the optically clear pressure-sensitive adhesives of Examples 1 to 5 with a dynamic shear strength as defined according to the present invention all achieved a significantly better peel adhesion performance than the comparative examples that are not according to the present invention at a high thickness of 500 pm and even 1000 pm,and all passed the rebound resistance test (i.e. no bubbles in appearance, and no delamination). This indicated that: these pressure-sensitive adhesives were able to maintain a good adhesive effect and appearance of the product in a bonded product for a long time; the adhesives had excellent mechanical properties (with a dynamic shear strength > 1 MPa) and bonding properties (with a peel adhesion > 20 N / cm); no rebound and no delamination; and at the same time, they also had good optical properties themselves, in particular low yellowing properties (characterized by a low Ab value / yellowing index after aging) and a good optical transparency. In addition, as compared with the comparative examples, the optically clear pressure-sensitive adhesives of Examples 1 to 5 according to the present invention showed no whitening (characterized by a haze value that was less than or equal to 1) after aging at both thicknesses, indicating that the optically clear pressure-sensitive adhesives according to the present invention still had a favorable optical stability at a higher thickness. Therefore, the pressure-sensitive adhesives according to the present invention were particularly suitable for applications in which a substrate with a 3D curved surface was bonded, such as for bonding display screens, smart car windows for automobile, or architectural glasses.Test methods

[0069] The tests to which the examples according to the present invention and comparative examples were subjected and their corresponding information were summarized in Table 2 below and would be described in detail below.

[0070] Table 2: Tests to which the examples according to the present invention and comparative examples were subjectedIn Table 2, "RH" denotes relative humidityI. Refractive index test

[0071] After the test sample was placed at a condition of 23 ± 1 °C and 50 ± 5% RH for 24 hours, the refractive index Rl value of the test sample under a light source of 589 nm was tested utilizing a Metricon prism coupler (model: Metricon 2010 / M PRISM COUPLER).II. Ab, yellowing index (Yl) test:

[0072] Conditions for accelerated aging test (105 °C, 1000 h)

[0073] The release liner on one side of the obtained adhesive layer was peeled off, and then the adhesive layer was laminated onto a glass sheet with a thickness of 1 mm using a pressing roller to ensure that there were no impurity particles and bubbles in the bonding interface. Afterwards, the release liner on the other side of the adhesive layer was peeled off, and the resulting bondedsheet was placed into a vacuum bonding chamber, wherein the adhesive sheet was facing upward, and after another glass sheet with a thickness of 1 mm was covered, vacuum bonding was performed under the bonding conditions of below 200 Pa for 10 seconds. The resulting laminate was then subjected to de-bubbling. The de-bubbling conditions were 40 °C, 0.5 MPa, and for a time period of 30 minutes. The resulting laminate was left under conditions of 25°C and 50% relative humidity for 24 hours, and then was placed into an oven at 105°C. Ab value was measured at intervals of 250 hours / 500 hours / 1000 hours, respectively, utilizing BYK spectro- guide (purchased from BYK-GARDNER GMBH, GERMANY); and the yellowing index Yl was measured utilizing a UV-visible spectrophotometer. BYK spectro-guide was calibrated on a white slate through a blank glass sheet.III. Transmittance (T) and haze tests:

[0074] Conditions for accelerated aging test (85°C / 85% RH, 1000 h)

[0075] The laminates were manufactured with the method described above, except that the placing conditions were as follows: the resulting laminate was placed under conditions of 25°C and 50% relative humidity for 24 hours, and then placed into a temperature and humidity chamber at 85°C and 85% relative humidity for carrying out the experiment, and BYK haze-gard plus (purchased from BYK-GARDNER GMBH, GERMANY) was utilized to measure haze values and transmittances at intervals of 250 hours / 500 hours / 1000 hours, respectively. In general, excellent optical properties correspond to a haze value less than 1 and a transmittance greater than 99%.IV. 90° peel adhesion test

[0076] Peel adhesion was measured at 23°C±1°C and 50%±5% relative humidity.

[0077] Test samples were cut to a width of 20 mm and adhered to glass and polycarbonate (PC) substrates, respectively. The substrates were cleaned and conditioned prior to measurements. Then, the side facing away from the test substrate was lined with a plastic film such aspolyethylene terephthalate (PET) and polyvinyl chloride (PVC) of 50 pm or other thickness to prevent stretching or deformation of the adhesive layer of the test sample during testing. The test samples were then rolled onto the substrates in the following manners: rolling back and forth on the test samples for 5 times with a roller of 4 kg at a rolling rate of 10 m / min; after the rolling treatment, the test samples were placed under conditions of 25°C and 50% relative humidity for 24 hours; and then the test samples were inserted into a Zwick tensile tester for peel adhesion testing at an angle of 90°. The measurement of the peel adhesion was performed at a speed of 300 mm / min. The test result of each test sample was expressed by the average of three measurement results (in N / cm).V. Tests for dynamic shear strength:

[0078] Size of glass: 7cm* 3 cm *5mmSize of sample: 3cm*3cmTest speed: 50 mm / minTest temperature: 23 °C

[0079] A sodium calcium glass with a size of 7cm * 3cm * 5mm was subjected to plasma treatment, then adhered with an adhesive tape sample with a size of 3 cm * 3 cm, and then adhered under vacuum with another piece of plasma-treated glass. After being well adhered, it was subjected to de-bubbling at 80°C and 0.5 MPa for 45 minutes. After de-bubbling, the sample was placed at room temperature for 24 hours, and then two pieces of glasses were pulled apart at an angle of 180° at a speed of 50 mm / min, as shown in Fig. 1. The maximum force required for this was determined, and the result was in N / cm2.VI. Rebound resistance test:

[0080] As shown in Fig. 2, an optically clear pressure-sensitive adhesive tape was adhered to a glass with a size of 19cm* 4 cm and a thickness of 0.6 mm. The release liner on the other side was removed, and then was adhered to a glass with a size of 23cm* 4.5 cm and a thickness of 0.4mm. Then, it was installed onto a fixture with a curvature of 800 mm. Both sides of the glasses were fixed, and the adhesion of the adhesive tape to the glasses was tested in a bending state.

[0081] Test conditions: 105°C, 1000 h. Adhesive effect of the adhesive tape on the curved substrate under high temperature and high humidity conditions was observed. If there was no bubble in the appearance of the adhesive tape, and there was no de-bonding delamination caused by rebound and lifting at the edge bonding position, the result was recorded as "+", otherwise it was recorded

Claims

CLAIMS1. An optically clear pressure-sensitive adhesive, wherein the optically clear pressure-sensitive adhesive has a dynamic shear strength of at least 1 MPa, preferably of 1 MPa to 3 MPa as measured according to ASTM D1002; and a Ab 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 h.

2. The optically clear pressure-sensitive adhesive as claimed in claim 1, wherein the optically clear pressure-sensitive adhesive, when being made into a sheet with a thickness of not less than 500 pm, particularly of 1000 pm, still has a dynamic shear strength of at least 1 MPa, preferably of 1 MPa to 3 MPa as measured according to ASTM D1002; and a Ab 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 h.

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

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

5. The optically clear pressure-sensitive adhesive as claimed in any one of claims 1 to 4, wherein the optically clear pressure-sensitive adhesive is derived from a monomer composition comprising:a) greater than or equal to 25 wt%, preferably 25 to 45 wt%, and more preferably 30 to 35 wt% of at least one hydroxyl-containing acrylate monomer; b) greater than or equal to 30 wt%, preferably 30 to 45 wt%, and more preferably 35 to 40 wt% or 30 to 35 wt% of at least one monomer with a high refractive index; c) greater than or equal to 5 wt%, preferably 8 to 40 wt%, and more preferably 10 to 30 wt% of at least one high Tg monomer; and d) optionally at least one monomer, which is different from the monomers a), b) and c), and is copolymerizable with the monomers a), b) and c), based on the total weight of the monomer composition.

6. The optically clear pressure-sensitive adhesive as claimed in claim 5, wherein the monomer a) is a hydroxy Ci-Ce alkyl (meth)acrylate, and is preferably selected from the group consisting of 4- hydroxybutyl acrylate (4-HBA) and 2-hydroxyethyl acrylate (2-HEA), more preferably 4- hydroxybutyl acrylate (4-HBA).

7. The optically clear pressure-sensitive adhesive as claimed in claim 5, wherein the monomer b) is a monomer having a refractive index of greater than or equal to 1.510, preferably of 1.510 to 1.580, more preferably of 1.515 to 1.580 and containing at least one aromatic ring, preferably the monomer b) is a (meth)acrylate containing at least one aromatic ring, and is preferably selected from the group consisting of 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 clear pressure-sensitive adhesive as claimed in claim 5, wherein the monomer c) is a monomer without aromatic ring structure, which has a glass transition temperature Tg of greater than or equal to 15°C, preferably of 15°C to 200°C, preferably of 30°C to 200°C, more preferably of 50°C to 200°C, further preferably of 50°C to 180°C, and is preferably selected from the group consisting of tert-butyl acrylate (tBA), trimethylcyclohexyl acrylate (TMCHA), cyclohexyl acrylate (CHA), N-vinylpyrrolidone (NVP), acryloyl morpholine (ACMO),dimethylacrylamide (DMAA), diethylacrylamide (DEAA) and isobornyl acrylate (IBOA), more preferably tert-butyl acrylate (tBA), N-vinylpyrrolidone (NVP) and cyclohexyl acrylate (CHA).

9. An adhesive sheet, comprising the optically clear pressure-sensitive adhesive as claimed in any one of claims 1 to 8, wherein the adhesive sheet has a thickness of greater than or equal to 250 pm, preferably of greater than or equal to 500 pm, and more preferably of 500 to 1000 pm.

10. Use of the optically clear pressure-sensitive adhesive as claimed in any one of claims 1 to 8 or the adhesive sheet as claimed in claim 9 for bonding two substrates, preferably wherein one of the substrates is 3D curved, and the other one is flat, single curved or 3D curved.

11. The use as claimed in claim 10, wherein the substrates are glass substrates or polycarbonates.

12. The use as claimed in claim 10 or 11, wherein the substrates are display screens, smart car windows for automobile, or architectural glasses.

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