Optically clear adhesives are used in many applications, including electronic device and display assembly. Antistatic additives may be used to provide properties to these adhesives that avoid the buildup of static (and may cause harmful discharge).

Summary

In one aspect, the present description relates to an antistatic optically clear adhesive. The optically clear adhesive includes a polymeric optically clear adhesive and an N-ethyl pyridinium TFSI salt additive, where the N-ethyl pyridinium TFSI salt has a melting point greater than 25°C and where the antistatic optically clear adhesive including the polymeric optically clear adhesive and the N-ethyl pyridinium TFSI salt additive has a luminous transmission of greater than 89% according to ASTM D 1003-95.

Detailed Description

Antistatic additives may be added to optically clear adhesives to provide desirable antistatic properties. Such antistatic properties may improve handling and safety margins while assembling devices or in environments sensitive to static discharge. As static charges may attract dust, antistatic additives may improve the cleanliness of certain assembly operations. Various additives are known or described for potential applicability in adhesives, however, it has been expected that suitable additives have low melting points, and in any case are not solid at room temperature. Especially in the context of optically clear adhesives, the potential phase separation of the antistatic additive from other components of the antistatic adhesive composition may cause scattering interfaces that increase haze and impair luminous transmission. Additionally, antistatic additives that are solid at room temperature were assumed to have limited solubility and mobility within the bulk of the composition, therefore not being able to “bloom” to the surface of the adhesive composition effectively.

Surprisingly, the present description identifies a particular antistatic additive not meeting the previously understood low melting point conditions. This antistatic additive has good antistatic performance without compromising the optical clarity (and/or luminous transmission) of the antistatic optically clear adhesive.

Any suitable optically clear adhesive composition may be used. In some embodiments, the optically clear adhesive composition may be a liquid optically clear adhesive, a hot melt optically clear adhesive, a heat-activated optically clear adhesive, and/or a photocurable optically clear adhesive. Suitable liquid optically clear adhesives may include acrylic monomers or reactive oligomers based on acrylic monomers, or polyacrylate-based oligomers with curable functionality. Other suitable liquid optically clear or photocurable optically clear adhesives include polyvinylbutyral and polyurethane (meth)acrylate compositions including a photoinitiator. Other materials for suitable optically clear adhesives include poly(meth)acrylates and derived adhesives, thermoplastic polymers such as silicone or silicone polyureas, polyesters, polyurethanes and combinations thereof.

In some embodiments, the optically clear adhesive is a pressure sensitive adhesive. The pressure sensitive adhesive component can be any material that has pressure sensitive adhesive properties. Pressure sensitive adhesives are well known and have properties including the following: (1) aggressive and permanent tack, (2) adherence to a substrate with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength to be removed cleanly from the adherend. Furthermore, the pressure sensitive adhesive component may be a single pressure sensitive adhesive or the pressure sensitive adhesive can be a combination of two or more pressure sensitive adhesives.

Pressure sensitive adhesives that may be useful in particular applications include, for example, those based on natural rubbers, synthetic rubbers, styrene block copolymers, polyvinyl ethers, poly(meth)acrylates (including both acrylates and methacrylates), polyolefins, and silicones.

Optically clear pressure sensitive adhesives are generally acrylate-based pressure sensitive adhesives. However, silicone based pressure sensitive adhesives, rubber resin based pressure sensitive adhesives, block copolymer-based adhesives — especially those including hydrogenated elastomers — or vinylether polymer based pressure sensitive adhesives may also have optically clear properties.

Useful alkyl acrylates (i.e., acrylic acid alkyl ester monomers) include linear or branched monofunctional unsaturated acrylates or methacrylates of non-tertiary alkyl alcohols, the alkyl groups of which may have from 4 to 14 or from 4 to 12 carbon atoms.

In one embodiment, the pressure sensitive adhesive is based on at least one poly(meth)acrylate (e.g., is a (meth)acrylic pressure sensitive adhesive). Poly(meth)acrylic pressure sensitive adhesives are derived from, for example, at least one alkyl (meth)acrylate ester monomer such as, for example, isooctyl acrylate, isononyl acrylate, 2-methyl-butyl acrylate, 2- ethyl-n-hexyl acrylate and n-butyl acrylate, isobutyl acrylate, hexyl acrylate, n-octyl acrylate, n- octyl methacrylate, n-nonyl acrylate, isoamylacrylate, n-decyl acrylate, isodecyl acrylate, isodecyl methacrylate, isobornyl acrylate, 4-methyl-2-pentyl acrylate and dodecyl acrylate; and at least one optional co-monomer component such as, for example, (meth)acrylic acid, vinyl acetate, N-vinyl pyrrolidone, (meth)acrylamide, a vinyl ester, a fumarate, a styrene macromer, alkyl maleates and alkyl fumarates (based, respectively, on maleic and fumaric acid), or combinations thereof.

In certain embodiments, the poly(meth)acrylic pressure sensitive adhesive is derived from between about 0 and about 20 weight percent of acrylic acid and between about 100 and about 80 weight percent of at least one of isooctyl acrylate, 2-ethyl-hexyl acrylate or n-butyl acrylate composition. One specific embodiment includes from between about 2 and about 10 weight percent acrylic acid and between about 90 and about 98 weight percent of at least one of isooctyl acrylate, 2-ethyl-hexyl acrylate or n-butyl acrylate composition. One specific embodiment is derived from about 2 weight percent to about 10 weight percent acrylic acid, about 90 weight percent to about 98 weight percent of isooctyl acrylate. Additional embodiments include about 70% by weight phenoxy ethyl acrylate and between about 25% by weight and about 30% by weight isooctyl acrylate. Such an embodiment may additionally contain between about 1% by weight and about 5% by weight acrylic acid.

The pressure sensitive adhesive may be inherently tacky. If desired, tackifiers may be added to a base material to form the pressure sensitive adhesive. Useful tackifiers include, for example, rosin ester resins, aromatic hydrocarbon resins, aliphatic hydrocarbon resins, and terpene resins. Other materials can be added for special purposes, including, for example, oils, plasticizers, antioxidants, ultraviolet (“UV”) stabilizers, hydrogenated butyl rubber, pigments, curing agents, crosslinking agents, polymer additives, thickening agents, chain transfer agents and other additives provided that they do not reduce the optical clarity of the pressure sensitive adhesive.

An antistatic agent operates by removing static charge or by preventing buildup of such charge. Adhesives delivered on one or more protective, removable liners (formed from or including silicone, plastics, paper, or any other suitable material) may also contribute to the buildup of static charge through triboelectric generation. Antistatic agents may include either polymeric (with repeat units) and non-polymeric organic salts. Generally, the antistatic agent may be present in any amount, compared to the optically clear adhesive. In some embodiments, the antistatic additive loading may be between 0.05 wt% and 10 wt%. Depending on the application, the particular loading of the antistatic additive may be selected to provide a balance of optical properties (as increased loading may provide decreased optical clarity) and cost against the appropriate or required antistatic performance.

In some embodiments, the antistatic agent is an ionic salt. Specifically, certain ionic salts described in U.S. Patent No. 6,372,829 (Lamanna et al.) may be particularly suitable. Described therein are ionic salts containing non-polymeric nitrogen-onium cations and a weakly coordinating fluoroorganic (either fully fluorinated/perfluorinated or partially fluorinated) anion. The nitrogen onium cation can be cyclic (that is, where the nitrogen atom(s) of the cation are ring atoms) or acyclic (that is, where the nitrogen atom(s) of the cation are not ring atoms but can have cyclic substituents). The cyclic cations can be aromatic, saturated, or can possess degrees of saturation and the acyclic cations can be saturated or unsaturated. The cyclic cations may comprise ring heteroatoms other than nitrogen (for example, oxygen or sulfur), and the ring atoms can bear substituents (for example, hydrogen, halogen, or organic groups such as alkyl, alicyclic, aryl, alkalicyclic, alkaryl, alicyclicalkyl, aralkyl, aralicyclic, and alicyclicaryl groups). Separate alkyl substituents can be joined together to constitute a unitary alkylene radical of from 2 to 4 carbon atoms forming a ring structure converging on nitrogen.

Of particular interest are salts that include the non-polymeric N-alkyl pyridinium cation. In some embodiments, these salts of N-alkyl pyridinium cations may have a melting point above 25°C. Exemplary N-alkyl pyridinium cations include N-ethyl pyridium and N-butyl pyridinium. For example, a suitable N-alkyl pyridinium cation salt is N-ethylpyridinium bis(trifluoromethanesulfonyl)imide: [ N-C 2 FF- yoN 5 FF ] [ NISCbCFrri]. As the anion in this salt may be referred to as TFSI, such salts may be described as N-ethyl pyridinium TFSI salts. N- butyl pyridinium TFSI salts may also be useful in certain applications.

In some embodiments, these N-ethyl pyridinium TFSI salts have a melting point greater than 25°C (approximately 31°C). N-butyl pyridinium TFSI salts also have a melting point greater than 25°C in some embodiments (approximately 26°C). Surprisingly, antistatic optically clear adhesives including such salts achieve antistatic effectiveness without unduly impairing the visible light luminous transmission of the antistatic optically clear adhesive. In some embodiments, the luminous transmission measured according to ASTM D 1003-95 is greater than 80%. In some embodiments, the luminous transmission measured according to ASTM D 1003-95 is greater than 85%. In some embodiments, the luminous transmission measured according to ASTM D 1003-95 is greater than 89%. In some embodiments, the luminous transmission measured according to ASTM D 1003-95 is greater than 90%. In some embodiments, the luminous transmission measured according to ASTM D 1003-95 is greater than 91%. In some embodiments, the luminous transmission measured according to ASTM D 1003-95 is greater than 92%.

An alternative or additional way to characterize the lack of undesirable visible scattering is by the range of optical haze for visible light. In some embodiments, the antistatic optically clear adhesive has a haze of less than 10% according to ASTM D 1003-95. In some embodiments, the antistatic optically clear adhesive has a haze of less than 5% according to ASTM D 1003-95. In some embodiments, optical haze measured according to ASTM D 1003-95 is less than 2%. In some embodiments, optical haze measured according to ASTM D 1003-95 is less than 1%.

Practical requirements of electrostatic dissipation may vary based on the requirements and desired application. In some embodiments, the surface resistivity may be on the order of or lower than 10 ¹¹ W/sq. In some embodiments, the bulk resistivity may be on the order of or lower than 10 ¹¹ W-cm.

Examples

Unless otherwise noted or readily apparent from the context, all parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight.

Materials Used in the Examples

Preparation of N-ethylpyridinium N.N-bisftrifluoromethylsulfonyl iimide N-ethylpyridinium bromide (474.0 g) was charged to a 2L round bottom 3-necked flask equipped with an addition funnel, thermocouple probe, water cooled condenser, and a magnetic stirbar. An additional 474 grams DI water was added to the reaction flask to yield a 50% by wt. solution of N-ethylpyridinium bromide in water. 80% by wt. lithium N,N- bis(trifluoromethylsulfonyl) imide (867.0 g) aqueous solution was gradually charged to the N- ethylpyridinium bromide solution at 70°C. The reaction mixture was stirred at 70°C for 60 minutes and then transferred to separatory funnel. Two layers were observed and the aqueous layer was removed. The organic layer was washed three times with water. After washing, the product was dried under vacuum at 100°C. The identity of the product was confirmed by 'H and ¹⁹ F NMR spectroscopy. Preparation of N-butylpyridinium N.N-bisCtrifluoromethyl sulfonyl )imide

Step 1 : Pyridine, 83.06 grams (5 mole% excess), was charged to a 500 mL three neck flask. The flask was equipped with an addition funnel, thermocouple probe, water cooled condenser connected to an N2 inlet, and a magnetic stirbar. 1-butylbromide, 137.02 grams, was charged to the addition funnel and gradually added to the reaction flask at 100°C. After a total of 4 hours reaction time, 212 ml D1 water was slowly added to the hot reaction mixture to yield 50% by wt. butylpyridinium bromide aqueous solutions at 100°C with stirring.

Step 2: 352.29 grams of 80% by wt. lithium N,N-bis(trifluoromethylsulfonyl) imide aqueous solution was added to a 1L round bottom 3-necked flask equipped with an addition funnel, thermocouple probe, water cooled condenser, and a magnetic stirbar. An additional 211 grams DI water was added to the reaction flask to yield a 50% by wt solution of lithium N,N- bis(trifluoromethylsulfonyl) imide in water. The entire 50% by wt butylpyridinium bromide aqueous solutions prepared in step 1 (432.08 grams) was transferred to the addition funnel and gradually charged to lithium N,N-bi s(trifluorom ethyl sulfonyl) imide solution at 70°C. The reaction mixture was stirred at 70°C for 60 minutes and then transferred to a separatory funnel. Two layers were observed and aqueous layer was removed. The organic layer was washed three times with water. After washing, the product was dried under vacuum at 100°C. The identity of the product was confirmed by 'H and ¹⁹ F NMR spectroscopy.

Preparation of N-octylpyridinium N.N-bisftrifluoromethyl sulfonyl limide

N-octylpyridinium bromide (54.45g) was charged to a round bottom 3 -necked flask equipped with an addition funnel, thermocouple probe, water cooled condenser, and a magnetic stirbar. An additional 54.45 grams DI water was added to the reaction flask to yield a 50% by wt solution of N-octylpyridinium bromide in water. 80% by wt. lithium N,N- bis(trifluorom ethyl sulfonyl) imide (71.77.0g) aqueous solution was gradually charged to the N- octylpyridinium bromide solution at 70°C. The reaction mixture was stirred at 70°C for 60 minutes and then transferred to separatory funnel. Two layers were observed and aqueous layer was removed. The organic layer was washed three times with water. After washing, the product was dried under vacuum at 100°C. The identity of the product was confirmed by 'H and ¹⁹ F NMR spectroscopy.

Test Methods

Surface Resistivity The surface resistivity of film samples was measured using a HIRESTA MCP-HT450 Surface and Volume High Resistivity Tester with a UR- 100 probe, both available from Mitsubishi Chemical Company (Tokyo, Japan). For each example and comparative example, two film samples were prepared. Using a voltage setting of 500 volts, the surface resistivity was measured at two points on each film sample, except CE-4 where a voltage of 1000 V was used due to the higher surface resistivity. The surface resistivity for a given example or comparative example was the average of the four measurements. Results are shown in Table 1.

Example 1 (Ex 13

A salt having a melting point of 31°C, N-ethylpyridinium N, Ti bi s(trifluorom ethyl sulfonyl)imide was added to an acrylic based adhesive solution available under the trade designation HWR-6122 from Hanwool Chemical Co., Ltd. (Ulsan-si, Republic of Korea). The adhesive solution was 34.84% solids and the salt was added to the adhesive solution at 1 weight percent, based on the weight of solids in the adhesive solution. The resulting solution was mixed and then hand coated on a conventional PET film using a coating bar. The film was thermally cured at 120°C for 2 minutes, followed by a drying step at 70°C for 48 hours. Two films were prepared following the above procedure

Example 2 (Ex 23

Ex. 2 was prepared similarly to Ex. 1 , except the amount of salt added to the adhesive solution was 3 weight percent, based on the weight of solids in the adhesive solution.

Example 3 (Ex. 3)

Ex. 3 was prepared similarly to Ex. 1, except the amount of salt added to the adhesive solution was 5 weight percent, based on the weight of solids in the adhesive solution.

Comparative Example 4 (CE-4)

CE-4 was prepared similarly to Ex. 1, except no salt was added to the adhesive solution. Comparative Example 5 (CE-5)

CE-5 was prepared similarly to Ex. 1, except the N-ethylpyridinium N,N- bis(trifluorom ethyl sulfonyl)imide salt was replaced by N-octylpyridinium N,N- bis(trifluorom ethyl sulfonyl)imide, a salt having a melting point of -6°C. N-octylpyridinium N,N- bis(trifluorom ethyl sulfonyl)imide was added to an acrylic based adhesive solution at 1 weight percent, based on the weight of solids in the adhesive solution. Comparative Example 6 (CE-6)

CE-6 was prepared similarly to CE-5, except the N-octylpyridinium N,N- bis(trifluorom ethyl sulfonyl)imide salt was added to an acrylic based adhesive solution at 3 weight percent, based on the weight of solids in the adhesive solution.

Comparative Example 7 (CE-7)

CE-7 was prepared similarly to CE-5, except the N-octylpyridinium N,N- bis(trifluorom ethyl sulfonyl)imide salt was added to an acrylic based adhesive solution at 5 weight percent, based on the weight of solids in the adhesive solution.

Example 8 (Ex. 8)

Ex. 8 was prepared similarly to Ex. 1, except the N-ethylpyridinium N,N- bis(trifluorom ethyl sulfonyl)imide salt was replaced by N-butylpyridinium N,N- bis(trifluorom ethyl sulfonyl)imide, a salt having a melting point of 26°C. N-butylpyridinium N,N-bis(trifluoromethylsulfonyl)imide was added to an acrylic based adhesive solution at 1 weight percent, based on the weight of solids in the adhesive solution.

Example 9 (Ex. 9)

Ex. 9 was prepared similarly to Ex. 8, except the N-butylpyridinium N,N- bis(trifluorom ethyl sulfonyl)imide salt was added to an acrylic based adhesive solution at 3 weight percent, based on the weight of solids in the adhesive solution.

Example 10 (Ex. 10)

Ex. 10 was prepared similarly to Ex. 8, except the N-butylpyridinium N,N- bis(trifluorom ethyl sulfonyl)imide salt was added to an acrylic based adhesive solution at 5 weight percent, based on the weight of solids in the adhesive solution.

Using the Surface Resistivity test method, the surface resistivity was measured for each example and comparative example, Table 1.

Table 1.