UV (ultraviolet) adhesive for difficult-to-adhere materials

Abstract

The invention discloses a UV (ultraviolet) adhesive for a difficult-to-adhere material, which is prepared from the following components in parts by weight: 100 parts of acrylic acid modified polydimethylsiloxane or chain-extended acrylic acid modified polydimethylsiloxane, 10 to 80 parts of acrylate monomer and 0.5 to 5.0 parts of free radical photoinitiator. The UV-cured acrylic acid modified organic silicon adhesive has excellent bonding strength on a silica gel cap and a PET (polyethylene terephthalate) film, can reach the damage degree of a silica gel cap material, and meets the harsh requirements of a keyboard bonding process and bonding strength.

Description

Technical Field

[0001] The invention belongs to the field of ultraviolet (UV) curing adhesives, and in particular relates to a UV adhesive composition having good adhesion and bonding strength to difficult-to-bond materials. Background Art

[0002] Keyboards are essential components for desktop and laptop computers. During the keyboard manufacturing process, the bonding of the silicone dome and the flexible circuit board (usually PET film material) is one of the most important steps. Since both the silicone dome and the PET film have low surface energy, it is difficult for ordinary adhesives to achieve satisfactory bonding strength.

[0003] Common adhesives that can be used to bond silicone caps to PET films include instant adhesive, RTV silicone, addition-type thermosetting silicone, and so on.

[0004] Instant adhesive has no bonding strength for silicone caps and PET that have not been surface treated, but after using a suitable primer to treat the surface of the silicone cap and PET film, it can show excellent bonding strength and even achieve material destruction. However, there is usually a whitening problem during the curing process of instant adhesive, which causes white mist around the silicone cap and affects the appearance. The use of primers also increases the complexity of the process and environmental pollution. In addition, trace moisture in the air can trigger the anionic polymerization reaction of instant adhesive and solidify. The production process is only suitable for dispensing, and large-area screen printing cannot be performed, and high-efficiency keyboard production cannot be performed. Therefore, the instant adhesive solution is only suitable for repairing and bonding a small number of keyboards, and cannot be used for large-scale mass production. Another important factor that instant adhesive has not been widely used is that the cured material is too hard. The silicone cap material used for the keyboard is very flexible and needs to withstand more than 10 million knocks. If the adhesive is hard, it is easy to fall off during the keyboard knocking process.

[0005] RTV organic silicone adhesive can also bond silicone caps and PET films. The cured product is very flexible and can achieve good bonding strength without a primer, but the curing speed is very slow, usually requiring more than 24 hours to reach practical strength. In addition, because it is a moisture-curing type, the production process can only be dispensed, and large-scale screen printing cannot be performed, so high-efficiency large-scale keyboard mass production is impossible.

[0006] Silicone hydrogen addition type heat-curing organic silicone adhesive can have very good bonding strength to silicone caps and PET films without primer. The cured product has good toughness and can withstand long-term knocking tests. It is also suitable for screen printing process and can be produced efficiently. It is the mainstream technical route for bonding silicone caps and PET films. However, heat-curing organic silicone generally requires a high temperature of more than 120°C and a heating time of more than 60 minutes. If a two-component addition type organic silicone adhesive is used, an additional mixing process of the AB components is required. The process is complicated and time-consuming, and there are problems and risks such as poor mixing and waste treatment of excess residual adhesive after mixing. Excessive curing temperature will also damage the newly designed heat-sensitive keyboard components. Therefore, the market urgently needs a new adhesive system that can be cured in a short time with a single component, does not heat up during the curing process, does not require a primer process, and can be screen printed to replace the addition type heat-curing organic silicone adhesive, complete the upgrade, improve production efficiency, and reduce product defective rate. Summary of the invention

[0007] The invention aims to provide a UV curing adhesive for difficult-to-bond materials (keyboard silicone cap and PET film).

[0008] The technical solution to achieve the purpose of the present invention is:

[0009] A UV adhesive composition for difficult-to-bond materials, comprising, by weight, 100 parts of acrylic acid-modified polydimethylsiloxane or chain-extended acrylic acid-modified polydimethylsiloxane, 10 to 80 parts of acrylate monomers, and 0.5 to 5.0 free radical photoinitiator, wherein the structures of the acrylic acid-modified polydimethylsiloxane and chain-extended acrylic acid-modified polydimethylsiloxane are as follows:

[0010]

[0011] Wherein n=an integer from 40 to 2000.

[0012] Preferably, the acrylic acid-modified polydimethylsiloxane or the chain-extended acrylic acid-modified polydimethylsiloxane is prepared by the following steps:

[0013] (1) a hydrogen-terminated silicone oil and trimethylolpropane monoallyl ether are subjected to a hydrosilylation reaction in the presence of a platinum catalyst to generate a hydroxy-terminated silicone oil having two hydroxyl groups at each end;

[0014]

[0015] (2) a hydroxy-terminated silicone oil having two hydroxyl groups at each end and an excess of acrylic acid are subjected to an esterification and dehydration reaction in the presence of an acidic catalyst and a cyclohexane water-carrying agent, and then neutralized with sodium carbonate, washed with water for multiple times and dried to obtain acrylic acid-modified polydimethylsiloxane;

[0016]

[0017] (3) Adding octamethylcyclotetrasiloxane to acrylic acid-modified polydimethylsiloxane, heating to 80° C. in the presence of an acidic catalyst to carry out a chain extension and polymerization reaction, neutralizing with sodium carbonate after reaching equilibrium, washing with water for multiple times and drying, and removing unreacted octamethylcyclotetrasiloxane in vacuo to obtain a chain-extended acrylic acid-modified polydimethylsiloxane.

[0018] Specifically, in order to prevent acrylic acid from self-polymerizing during the acrylic esterification and dehydration reaction, a free radical inhibitor needs to be added. The free radical inhibitor is any one or more of p-methoxyphenol (MeHQ), hydroquinone, 2,5-dimethylhydroquinone, and 2,6-di-tert-butyl-4-methylphenol (BHT), and the amount added is 0.01% to 1% of the total mass of the reactants.

[0019] Specifically, the acidic catalyst used in the esterification dehydration reaction and the chain extension polyol reaction is selected from any one of concentrated sulfuric acid, sodium bisulfate, methanesulfonic acid, toluenesulfonic acid, trifluoromethanesulfonic acid, a strong acid ion exchange resin and a solid super acid, preferably any one of methanesulfonic acid, trifluoromethanesulfonic acid, a strong acid ion exchange resin and a solid super acid, and the added amount is 0.1% to 5.0% of the total mass of the reactants.

[0020] Preferably, the acrylate monomer is selected from any one or two of isobornyl acrylate, cyclohexyl acrylate, tetrahydrofuran acrylate, tricyclodecyl acrylate, dicyclopentadiene acrylate, acryloylmorpholine and N,N-dimethylacrylamide, preferably isobornyl acrylate.

[0021] Preferably, the free radical photoinitiator is selected from 2-hydroxy-2-methylphenylacetone (Darocure 1173), 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO), 2,4,6-trimethylbenzoylphenylphosphonic acid ethyl ester (TPO-L), a mixture of 2,4,6-trimethylbenzophenone and 4-methylbenzophenone, benzoin dimethyl ether, benzophenone (BP), 1-hydroxy-cyclohexyl benzophenone, α,α′-ethoxyacetophenone (DEAP) or α-aminoalkylphenone, or a mixture of several thereof.

[0022] Preferably, based on the consideration of water resistance, the UV adhesive composition for the difficult-to-bond material may also be added with various epoxy silane coupling agents, titanate coupling agents, zirconate coupling agents, aluminate coupling agents, etc.

[0023] Preferably, the UV adhesive composition for the difficult-to-bond material may also include hydrophobic silicone.

[0024] Compared with the prior art, the advantages of the present invention are:

[0025] (1) The UV-curable acrylic modified organic silicone adhesive of the present invention has the advantages of single component, no need to mix AB components, can be screen printed, and has a short curing time (curing in seconds). It can replace the addition type heat-curing organic silicone adhesive, greatly shorten the engineering time, and improve the production efficiency.

[0026] (2) The UV-curable acrylic modified organic silicone adhesive of the present invention has excellent bonding strength to the silicone cap and the PET film, which can reach the level of silicone cap material destruction, meeting the stringent requirements of keyboard bonding process and bonding strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the bonding structure of the silicone cap / PET film of the present invention. DETAILED DESCRIPTION

[0028] The present invention is described in detail below with reference to the embodiments.

[0029] An effective method for inserting a (meth)acryloyloxy group at the end of polydimethylsiloxane is to utilize the hydrosilylation reaction. First, the end-side hydrogen-containing silicone oil (polydimethylsiloxane) and a bifunctional compound having an allyl group and an epoxy group or a hydroxyl group are subjected to hydrosilylation to generate a polydimethylsiloxane having an end-side epoxy group or a hydroxyl group structure. The polydimethylsiloxane having an epoxy group or a hydroxyl group structure is then reacted with (meth)acrylic acid to introduce the (meth)acryloyloxy group into the end group or side chain of the polydimethylsiloxane, thereby imparting photocuring properties.

[0030] The present invention uses terminal hydrogen silicone oil (hydrogen-terminated polydimethylsiloxane) and trimethylolpropane monoallyl ether as raw materials, performs a silylation reaction in the presence of chloroplatinic acid or Custer catalyst to generate terminal hydroxy silicone oil (terminated hydroxy polydimethylsiloxane) containing two hydroxyl groups at both ends, and then performs an esterification dehydration reaction with excess acrylic acid in the presence of an acidic catalyst and a cyclohexane water-carrying agent. The water generated by the reaction is brought out by cyclohexane reflux to promote the reaction to proceed in the direction of the target product, and then the acidic catalyst is neutralized with sodium carbonate, and the reaction is washed with water and dried for multiple times to obtain a polydimethylsiloxane with two acryloyloxy groups at both ends, namely, acrylic acid-modified polydimethylsiloxane. The synthesis route is as follows:

[0031]

[0032] The generated acrylic modified polydimethylsiloxane can be further chain extended to increase the molecular weight and viscosity of the oligomer. Specifically, D4 ring body (octamethylcyclotetrasiloxane) is added to the acrylic modified polydimethylsiloxane, heated to 80°C in the presence of an acidic catalyst for chain extension and telomerization reaction, neutralized with sodium carbonate after reaching equilibrium, washed with water and dried multiple times, and the unreacted D4 ring body is removed under high vacuum and high temperature to obtain high molecular weight acrylic modified polydimethylsiloxane (chain extended acrylic modified polydimethylsiloxane). Since the chain extension and esterification dehydration both use the same acidic catalyst, after the esterification and dehydration reaction of the terminal hydroxyl silicone oil and acrylic acid is completed, the D4 ring body can also be directly added for chain extension and telomerization reaction.

[0033] Synthetic (chain-extended) acrylic modified polydimethylsiloxane is combined with acrylate monomers, free radical photoinitiators, coupling agents, and hydrophobic silica to form a UV adhesive with excellent bonding strength to silicone and PET films.

[0034] If the specific conditions are not specified in the examples, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. If the manufacturers of the reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0035] Acrylic acid modified synthesis example-1

[0036] 100 g of terminal hydrogenated silicone oil with a hydrogen content of 0.035% was added into a three-necked glass flask equipped with an oil bath and a condenser tube device, and the temperature of the terminal hydrogenated silicone oil in the flask was raised to 80°C. A mixed solution of 6.8 g of trimethylolpropane monoallyl ether and 0.01 g of chloroplatinic acid was added dropwise. The addition was completed within one hour, and then the temperature was raised to 110°C and the reaction was continued for 4 hours. After FTIR (Fourier transform infrared spectrometer) detection, the SiH stretching absorption peak of the terminal hydrogenated silicone oil corresponding to 2150 cm -1 The peak completely disappeared, indicating that the hydrosilylation reaction was completed, and the reaction was stopped to obtain a hydroxy-terminated silicone oil (hydroxy-terminated polydimethylsiloxane) containing two hydroxyl groups at each end. The product molecular weight was 6118.

[0037] In the above three-necked glass flask, add 106.8g of hydroxyl-terminated silicone oil, 13.5g of acrylic acid, 0.05g of free radical inhibitor MeHQ, 0.1g of trifluoromethanesulfonic acid catalyst, and 50.0g of cyclohexane, heat to 85°C and reflux, and release the water brought out regularly with a separatory tube, carefully observe the amount of water retained in the separatory tube until no more excess water is distilled out, and the reaction is considered complete. Cool to room temperature, add excess sodium carbonate to neutralize the residual acrylic acid and acidic catalyst, precipitate and filter, wash with water several times, and then vacuum dry to obtain acrylic acid modified polydimethylsiloxane with a molecular weight of about 6334. The product is named acrylic acid modified synthesis example-1.

[0038] Acrylic acid modified synthesis example-2

[0039] Add 100g of the above-mentioned acrylic acid modified synthesis example-1 into a three-necked glass flask with an oil bath and a condenser device, add 500g of D4 ring body (octamethylcyclotetrasiloxane), 1.2g of trifluoromethanesulfonic acid catalyst, react at 80°C for 10h, cool to room temperature, add excess sodium carbonate to neutralize the residual acid catalyst, precipitate and filter, wash with water several times, and then vacuum remove the unreacted D4 ring body to obtain an extended chain acrylic acid modified polydimethylsiloxane with a molecular weight of about 33,000. The product is named acrylic acid modified synthesis example-2.

[0040] Acrylic acid modified synthesis example-3

[0041] Add 100g of the above-mentioned acrylic acid modified synthesis example-1 into a three-necked glass flask with an oil bath and a condenser device, add 1200g of D4 ring body (octamethylcyclotetrasiloxane), 50g of solid superacid catalyst, react at 80°C for 10h, cool to room temperature, filter out the solid superacid catalyst, and then remove the unreacted D4 ring body in vacuum to obtain an extended chain acrylic acid modified polydimethylsiloxane with a molecular weight of about 70000. The product is named acrylic acid modified synthesis example-3.

[0042] The formulations of the embodiments and comparative examples are shown in Table 1 below.

[0043] Table 1

[0044]

[0045] Note: TS720 is Cabot hydrophobic silica from the United States; R970 is Evonik hydrophobic silica from Germany; KBM403 is γ-glycidyloxypropyltrimethoxysilane; KBM5103 is 3-(acryloyloxy)propyltrimethoxysilane; Baojun Chemical 2900 is a polyurethane type acrylate oligomer with good bonding strength to PET film; Zhongshan Qianyou 3001 is a polyurethane type acrylate oligomer with good bonding strength to PET film; IBOA is isobornyl acrylate; DMAA is N,N-dimethylacrylamide; ACMO is acryloylmorpholine; THFA is tetrahydrofuran acrylate.

[0046] Test method for bonding strength of silicone cap / PET film:

[0047] See the schematic diagram of the bonding structure of silicone cap / PET film. Figure 1 After screen printing on the PET film, a silicone cap is attached for UV curing. The thickness of the adhesive layer is controlled at about 50μm. The light source is a high-pressure mercury lamp with an irradiation dose of 2000mJ / cm 2The PET film is perforated so that after curing is completed, the thrust of the thrust gauge is inserted through the hole of the PET film and the thrust is applied from bottom to top to measure the thrust required to damage the adhesive or the silicone cap. 100 silicone caps are tested in each group and the thrust average is taken. The thrust unit is gram (g). Neither the PET film nor the silicone cap has undergone physical surface treatment such as plasma and corona, nor has it undergone chemical surface treatment with primer.

[0048] According to Table 1, by comparing Examples 1 to 6, it is found that the (chain-extended) acrylic modified polydimethylsiloxane UV adhesive compounded with acrylate monomers all show good bonding strength to the silicone cap / PET film. However, Comparative Example 1 uses a single acrylic modified polydimethylsiloxane without compounding with acrylate monomers, and its bonding strength is relatively low. Comparative Examples 2 to 4 use commercially available polyurethane acrylate oligomers compounded with UV adhesives that have good bonding strength to PET films. Although they have good adhesion to PET films, because the surface energy of the silicone cap is too low, the compounded UV adhesive cannot wet the surface of the silicone cap well, and the bonding strength is very low, and cannot be used for bonding for this purpose.

Claims

1. A UV adhesive composition for difficult-to-bond materials, characterized in that: The invention comprises, by weight, 100 parts of acrylic acid-modified polydimethylsiloxane or chain-extended acrylic acid-modified polydimethylsiloxane, 10 to 80 parts of acrylate monomer, and 0.5 to 5.0 parts of free radical photoinitiator, wherein the structures of the acrylic acid-modified polydimethylsiloxane and chain-extended acrylic acid-modified polydimethylsiloxane are as follows: Wherein n=an integer from 40 to 2000.

2. The UV adhesive composition for difficult-to-bond materials according to claim 1, wherein: The acrylic acid-modified polydimethylsiloxane and the chain-extended acrylic acid-modified polydimethylsiloxane are prepared by the following steps: (1) Hydrogen-terminated silicone oil and trimethylolpropane monoallyl ether undergo a hydrosilylation reaction in the presence of a platinum catalyst to generate a hydroxy-terminated silicone oil containing two hydroxyl groups at each end; (2) esterification and dehydration reaction of hydroxyl-terminated silicone oil containing two hydroxyl groups at each end and excess acrylic acid in the presence of an acidic catalyst and a cyclohexane water-carrying agent, neutralized with sodium carbonate, washed with water, and dried to obtain acrylic acid-modified polydimethylsiloxane; (3) Octamethylcyclotetrasiloxane is added to acrylic acid modified polydimethylsiloxane, and the mixture is heated to 80°C in the presence of an acidic catalyst to carry out a chain extension and polymerization reaction. After reaching equilibrium, the mixture is neutralized with sodium carbonate, washed with water and dried, and the unreacted octamethylcyclotetrasiloxane is removed in vacuo to obtain a chain extended acrylic acid modified polydimethylsiloxane.

3. The UV adhesive composition for difficult-to-bond materials according to claim 2, wherein: A free radical inhibitor is added during the esterification dehydration reaction. The free radical inhibitor is selected from any one or more of p-methoxyphenol (MeHQ), hydroquinone, 2,5-dimethylhydroquinone, and 2,6-di-tert-butyl-4-methylphenol (BHT), and the amount added is 0.01% to 1% of the total mass of the reactants.

4. The UV adhesive composition for difficult-to-bond materials according to claim 2, wherein: The acidic catalyst used in the esterification dehydration reaction and the chain extension polyol reaction is selected from any one of concentrated sulfuric acid, sodium bisulfate, methanesulfonic acid, toluenesulfonic acid, trifluoromethanesulfonic acid, a strong acidic ion exchange resin and a solid super acid, preferably any one of methanesulfonic acid, trifluoromethanesulfonic acid, a strong acidic ion exchange resin and a solid super acid, and the added amount thereof is 0.1% to 5.0% of the total mass of the reactants.

5. The UV adhesive composition for difficult-to-bond materials according to claim 1, wherein: The acrylate monomer is selected from any one or two of isobornyl acrylate, cyclohexyl acrylate, tetrahydrofuran acrylate, tricyclodecyl acrylate, dicyclopentadiene acrylate, acryloylmorpholine and N,N-dimethylacrylamide, preferably isobornyl acrylate.

6. The UV adhesive composition for difficult-to-bond materials according to claim 1, wherein: The free radical photoinitiator is selected from 2-hydroxy-2-methylphenylacetone (Darocure 1173), 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO), 2,4,6-trimethylbenzoylphenylphosphonic acid ethyl ester (TPO-L), a mixture of 2,4,6-trimethylbenzophenone and 4-methylbenzophenone, benzoin dimethyl ether, benzophenone (BP), 1-hydroxy-cyclohexyl benzophenone, α,α′-ethoxyacetophenone (DEAP) or α-aminoalkyl phenone, or a mixture of several thereof.

7. The UV adhesive composition for difficult-to-bond materials according to claim 1, wherein: The composition further comprises one or more coupling agents including epoxy silane coupling agents, titanate coupling agents, zirconate coupling agents and aluminate coupling agents.

8. The UV adhesive composition for difficult-to-bond materials according to claim 1, wherein: The composition also includes hydrophobic silica.

9. A UV adhesive for difficult-to-bond materials prepared from the composition according to any one of claims 1 to 8.

Images