Photo-curing organic silicon and UV (ultraviolet) adhesive composition for difficult-to-adhere materials

Abstract

A preparation method of the UV adhesive composition comprises the following steps: carrying out a hydrosilylation reaction on hydrogen-containing double end caps and AGE in the presence of a platinum catalyst to obtain an epoxy group double end cap agent, then carrying out a ring-opening esterification reaction on the epoxy group double end cap agent and excessive acrylic acid in the presence of a catalyst to obtain a hydroxyl acryloyloxy group double end cap agent, and finally, carrying out a reaction on the hydroxyl acryloyloxy group double end cap agent and the hydroxyl acryloyloxy group double end cap agent to obtain the UV adhesive composition. The preparation method comprises the following steps: carrying out an esterification dehydration reaction with excessive acrylic acid in the presence of a cyclohexane water-carrying agent and an acidic catalyst to obtain a bisacryloyloxy double-end-capping agent, and finally reacting with polydimethylsiloxane under the action of a linear phosphonium chloride catalyst to synthesize the light-cured organosilicon oligomer. The low polymer is matched with an acrylate monomer and a free radical photoinitiator to form the UV adhesive, and the UV adhesive has excellent bonding strength on low-surface-energy substances.

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] The bonding of low surface energy materials (such as PE, PP, PET, POE, etc.) has always been a difficult problem and pain point in the adhesive industry. These materials are generally considered to be difficult to bond. Although hot melt adhesives have been developed for these low surface energy materials, hot melt adhesives themselves are thermoplastic resins, and their use temperature is difficult to exceed 100°C, which greatly limits their application in the electronic and electrical industry that requires heat resistance.

[0003] UV glue (ultraviolet curing adhesive) is increasingly favored by the industry because of its fast curing speed, energy saving and environmental protection, especially in the assembly line of electronic components. UV glue can be cured in seconds, the assembly speed of the production line can be greatly accelerated, and the use of UV glue can greatly improve production efficiency. The oligomers used in UV glue usually have more than two functional groups, can form a cross-linked network, and have good heat resistance. Therefore, UV glue is used for potting and bonding of many electronic components. However, UV glue composed of ordinary polyurethane acrylate compounds does not have sufficient bonding strength for low surface energy materials, and the cationic curing system of alicyclic epoxy resin does not have good adhesion to these materials. To obtain good bonding strength, the surface of the material must be physically treated (mechanical grinding and roughening of the surface, short-wavelength ultraviolet irradiation, corona, plasma treatment, etc.) or chemically treated (silane coupling agent, primer, etc.). Sometimes, even if the surface treatment is performed, sufficient bonding strength cannot be obtained. These treatment projects are not only time-consuming and labor-intensive, but also increase the process cost and pollute the working environment. Therefore, the industry has been looking for a UV glue that can be directly applied without surface treatment.

[0004] Japanese patent (Japanese Patent No. 2008-31307) discloses a photocurable organosiloxane composition, which has good bonding strength to PET, PBT, PC, ABS, and LCP (liquid crystal polymer). The synthesis method is to use terminal hydroxyl polydimethylsiloxane (107 silicone oil) and aminosilane coupling agent for dealcoholization reaction, and then react with 2-isocyanatoethyl methacrylate to form methacryloyloxy-terminated polydimethylsiloxane, and combine with some acrylate monomers compatible with organosilicon to form UV glue. The shear strength of PET / PET can reach 2.8MPa, which basically meets the bonding strength requirements of most applications. However, the price of this 2-isocyanatoethyl methacrylate monomer is very expensive, resulting in the high price of the synthesized methacryloyloxy-terminated polydimethylsiloxane, which can only be used for high-end applications. In addition, this methacryloyloxy-terminated polydimethylsiloxane has a fatal flaw. After long-term storage, it will undergo trace hydrolysis to produce a foul-smelling substance. Customers will feel very uncomfortable during use, and no effective solution has been found so far. Summary of the invention

[0005] The purpose of the present invention is to provide a light-curing organic silicone adhesive for difficult-to-bond materials (such as PE, PP, PET, POE, etc.).

[0006] The technical solution to achieve the purpose of the present invention is:

[0007] In a first aspect, the present invention provides a photocurable silicone, which is an acrylic modified polydimethylsiloxane having the following structure:

[0008]

[0009] Wherein n=an integer from 40 to 2000.

[0010] In a second aspect, the present invention provides a method for synthesizing the photocurable silicone according to the first aspect, comprising:

[0011] (1) a step of performing a hydrosilylation reaction with a hydrogen-containing double-capping agent (tetramethyldisiloxane) and allyl glycidyl ether (AGE) in the presence of a platinum catalyst to generate a double-capping agent having an epoxy group at each end,

[0012]

[0013] (2) a step of subjecting a double-capping agent having an epoxy group at each end and an excess of acrylic acid to a ring-opening esterification reaction in the presence of a ring-opening esterification catalyst to obtain a double-capping agent having an acryloyloxy group and a hydroxyl group at each end,

[0014]

[0015] (3) subjecting a double-capping agent having an acryloyloxy group and a hydroxyl group at each end to an esterification and dehydration reaction with excess acrylic acid in the presence of a cyclohexane water-carrying agent and an acidic catalyst to generate a double-capping agent having two acryloyloxy groups at each end,

[0016]

[0017] (4) a step of rearranging a double-capping agent having two acryloyloxy groups at each end with polydimethylsiloxane in the presence of a linear chlorinated phosphine catalyst to obtain a high molecular weight acrylic acid-modified polydimethylsiloxane, i.e., a photocurable silicone.

[0018]

[0019] Preferably, in order to prevent acrylic acid from self-polymerizing during the ring-opening esterification and esterification dehydration reactions, a free radical inhibitor needs to be added in both the ring-opening esterification and esterification dehydration reactions, and the added amount is 0.01% to 1% of the total mass of the reactants.

[0020] Specifically, the free radical inhibitor is any one or more of p-methoxyphenol (MeHQ), hydroquinone (HQ), and 2,6-di-tert-butyl-4-methylphenol (BHT).

[0021] Preferably, the ring-opening esterification catalyst is selected from tertiary amines and quaternary ammonium salts, and is commonly used any one of triethylamine, N,N-dimethylbenzylamine, N,N-dimethylaniline, trimethylbenzylammonium chloride, triphenylphosphine, triphenylantimony, chromium acetylacetonate, chromium isooctanoate, stannous octoate, tetraethylammonium bromide, etc., and its usage is 0.1% to 5.0% of the reactants.

[0022] Preferably, the acidic catalyst 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 superacid, preferably any one of methanesulfonic acid, trifluoromethanesulfonic acid, a strong acid ion exchange resin and a solid superacid, and the added amount is 0.1% to 5.0% of the total mass of the reactants.

[0023] Preferably, the amount of linear phosphonium chloride catalyst added is 0.005-1.0% of the total amount of reactants.

[0024] Preferably, after the rearrangement reaction is completed, a passivating agent is used to inactivate the linear phosphonium chloride, and the amount of the passivating agent added is 1.1 to 4.0 times the amount of the linear phosphonium chloride added.

[0025] Specifically, the passivating agent can be selected as follows: First, amine compounds, such as triethylamine, propylamine, triethylnonylamine, hexamethyldisilazane, hexamethylcyclotrisilazane, etc. Second, lithium salts, n-butyl lithium n-hexane solution, hexamethyldisilazane lithium salt, trimethylsiloxy lithium, polydimethylsiloxane alkali glue of lithium hydroxide, etc. can be selected. Third, epoxy compounds, compounds containing more than one epoxy group in the molecule such as propylene oxide, glycidyl ether, bisphenol A diglycidyl ether, phenyl glycidyl ether, cyclohexene oxide, etc. can be selected. Fourth, salts of magnesium, aluminum, calcium, zinc, barium, and sodium, magnesium oxide, magnesium carbonate, magnesium hydroxide, aluminum oxide, zinc oxide, barium carbonate, sodium carbonate, etc. are selected. Triethylamine and hexamethyldisilazane are preferred.

[0026] In a third aspect, the present invention provides a UV adhesive composition for difficult-to-bond materials comprising the photocurable silicone described in the first aspect, which comprises, by weight, 100 parts of photocurable silicone, 10 to 80 parts of acrylate monomers and 0.5 to 5.0 parts of free radical photoinitiator.

[0027] Preferably, the acrylate monomer is selected from any one or two of isobornyl acrylate (IBOA), cyclohexyl acrylate, tetrahydrofuranyl acrylate (THFA), tricyclodecyl acrylate, dicyclopentadiene acrylate, acryloyl morpholine (ACMO) and N,N-dimethylacrylamide (DMAA), preferably isobornyl acrylate, tetrahydrofuranyl acrylate (THFA), acryloyl morpholine (ACMO) and N,N-dimethylacrylamide (DMAA).

[0028] 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.

[0029] 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.

[0030] Preferably, the UV adhesive composition for difficult-to-bond materials further includes hydrophobic silica.

[0031] Compared with the prior art, the advantages of the present invention are:

[0032] (1) The present invention adopts a linear chlorinated phosphinothricin catalyst, and carries out a rearrangement reaction between the bisacryloxy double-capped and polydimethylsiloxane in the presence of the linear chlorinated phosphinothricin catalyst to obtain a high molecular weight photocurable silicone. The reaction conditions are mild, the reaction speed is relatively fast, and no siloxane ring body is produced during the entire reaction process, and the volatile matter is extremely small, which well solves the problems of low activity of the target product of the traditional acid-catalyzed equilibrium reaction, low content of effective ingredients, residual low-boiling substances and many ring bodies. The cumbersome process of high vacuum and long-term high-temperature devolatilization with nitrogen gas in the later stage of the reaction is eliminated.

[0033] (2) The photocurable organic silicone adhesive composition of the present invention has excellent bonding strength to low surface energy materials (such as PE, PP, PET, POE, etc.). DETAILED DESCRIPTION

[0034] The present invention is described in detail below with reference to the embodiments.

[0035] An effective method for connecting (meth)acryloyloxy groups to both ends of polydimethylsiloxane is to utilize the hydrosilylation reaction. First, terminal hydrogen-containing silicone oil 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 a terminal epoxy group or a hydroxyl group structure. Then, the polydimethylsiloxane having a terminal epoxy group or a hydroxyl group structure is reacted with (meth)acrylic acid to introduce the (meth)acryloyloxy groups into the terminal groups of the polydimethylsiloxane, thereby making it have photocuring properties.

[0036] The purpose of the present invention is to connect two acryloyloxy functional groups to each end of polydimethylsiloxane, so that its photocurability is better than that of polydimethylsiloxane with one acryloyloxy structure at each end. In particular, for high molecular weight polydimethylsiloxane, if there is only one acryloyloxy group at the end group, the photocurability will be weakened due to the very small proportion of photocurable functional groups. If there are two acryloyloxy groups at each end group, the photocurability will be greatly improved, the crosslinking density will be higher, and the bonding strength will be correspondingly greater.

[0037] If a terminal hydrogen silicone oil (high molecular weight terminal hydrogen silicone oil) with extremely low hydrogen content is directly used as the initial reaction raw material and allyl glycidyl ether (AGE) is used to synthesize a high molecular weight terminal functional silicone oligomer, the SiH content is too low, resulting in an incomplete silane addition reaction, that is, less active substances are connected to the terminal group, which is not conducive to the next step of the reaction to obtain a silicone oligomer with good photocurability. In addition, allyl glycidyl ether (AGE) and low hydrogen silicone oil (that is, high molecular weight terminal hydrogen silicone oil) have poor compatibility, and the reaction is difficult to carry out. The common method in the industry is to use the silane addition reaction to synthesize a low molecular weight terminal active polydimethylsiloxane, and then extend the chain through the D4 equilibrium polymerization reaction to obtain a high molecular weight terminal active polydimethylsiloxane. However, there are disadvantages such as low equilibrium conversion rate, the need to remove low boiling point residues and small molecular ring bodies after the reaction is completed, poor yield, and complicated process.

[0038] The synthesis method of the present invention is to first synthesize a double-capping agent bisacryloxy group with two acryloxy groups at both ends, and then carry out a rearrangement reaction with polydimethylsiloxane in the presence of a linear chlorinated phosphinate catalyst to obtain a high molecular weight light-cured polydimethylsiloxane. During the entire reaction process, no siloxane ring body is generated, and the volatile matter is extremely small, which well solves the problems of low activity of the target product of the traditional acid-catalyzed equilibrium reaction, low content of effective components, residual low-boiling substances and many ring bodies. After the reaction is completed, a passivating agent is used to inactivate the linear chlorinated phosphinate.

[0039] Synthetic light-curing silicone is combined with acrylate monomers, free radical photoinitiators, coupling agents, and hydrophobic silicone to form a UV adhesive with excellent bonding strength to low surface energy materials.

[0040] 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.

[0041] The following is a detailed introduction to the synthesis and formulation examples of photocurable polydimethylsiloxane.

[0042] Synthesis of epoxy double capping agent.

[0043] Into a three-necked glass flask equipped with an oil bath and a condenser, 160 g of allyl glycidyl ether (AGE) and 0.01 g of chloroplatinic acid were added, the temperature was raised to 70°C with stirring, and 100 g of hydrogen-containing double-capping agent (1,1,3,3-tetramethyldisiloxane) was added dropwise. The addition was completed within one hour, and then the temperature was raised to 80°C and the reaction was continued for 4 hours. The unreacted hydrogen-containing double-capping agent was removed in vacuo to obtain a colorless, transparent double-capping agent containing an epoxy group at each end (referred to as epoxy double-capping agent) with a molecular weight of 362.7.

[0044] Synthesis of hydroxyacryloyloxy double capping agent.

[0045] Into a three-necked glass flask equipped with an oil bath and a condenser device, 100 g of epoxy double-capping agent, 0.5 g of triethylamine, and 0.05 g of MeHQ free radical inhibitor were added. The temperature was raised to 70°C with stirring, and 50 g of acrylic acid was added dropwise. The addition was completed within one hour, and then the temperature was raised to 80°C and the reaction was continued for 6 hours. Excessive sodium carbonate was added to neutralize the residual acrylic acid and triethylamine. The precipitate was filtered, washed with water several times, and then vacuum dried to obtain a light yellow transparent double-capping agent containing a hydroxyl group and an acryloyloxy group at each end (referred to as hydroxyacryloyloxy double-capping agent) with a molecular weight of 506.8.

[0046] Synthesis of bisacryloyloxy double capping agent.

[0047] Add 100g of hydroxyacryloxy double-capping agent, 0.05g of MeHQ free radical inhibitor, 0.05g of BHT free radical inhibitor, 0.5g of trifluoromethanesulfonic acid, and 100g of cyclohexane water-carrying agent into a three-necked glass flask with an oil bath and a condenser, stir and heat to 85°C and reflux, add 35g of acrylic acid dropwise, and complete the addition within one hour. The water brought out is regularly released with a separatory tube, and the amount of water retained in the separatory tube is carefully observed 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 trifluoromethanesulfonic acid, precipitate and filter, wash with water several times, and then vacuum dry to obtain a light yellow transparent double-capping agent with two acryloyloxy groups at each end (referred to as bisacryloxy double-capping agent), with a molecular weight of 614.9.

[0048] Modified silicone synthesis example-1.

[0049] 85g of polydimethylsiloxane with a viscosity of 1000mPas and 0.1g of linear phosphine chloride catalyst were added to a three-necked glass flask with an oil bath and a condenser. The temperature was raised to 80°C under nitrogen protection with stirring. 10g of bisacryloxy double-capping agent was gradually added dropwise. The addition was completed within one hour. The temperature was then raised to 110°C and the reaction was continued for 4h. The temperature was lowered to room temperature, and 0.2g of triethylamine passivator was added to obtain a colorless and transparent polydimethylsiloxane containing two acryloxy groups at each end, i.e., a light-cured silicone with a molecular weight of about 6520. The product was named Modified Silicone Synthesis Example-1.

[0050] Modified silicone synthesis example-2.

[0051] In a three-necked glass flask with an oil bath and a condenser, add 1000g of polydimethylsiloxane with a viscosity of 500mPas and 0.5g of linear chlorinated phosphine catalyst, stir and heat to 80°C, gradually add 19g of double-capping agent bisacryloxy, and complete the addition within half an hour, then heat to 110°C and continue to react for 4h, cool to room temperature, add 1.0g of hexamethyldisilazane passivator, and obtain colorless and transparent polydimethylsiloxane with two acryloxy groups at each end, i.e., light-cured silicone, with a molecular weight of about 36900. The product is named Modified Silicone Synthesis Example-2.

[0052] The light-curable silicone prepared above is compounded with acrylate monomers and the like to prepare a UV adhesive composition. The specific formula is shown in Table 1.

[0053] Table 1

[0054] Example-1 Example-2 Example-3 Example-4 Comparative Example-1 Comparative Example-2 Comparative Example-3 Comparative Example-4 Modified silicone synthesis example-1 100 100 Modified silicone synthesis example-2 100 100 100 Baojun Chemical Industry 2900 100 Zhongshan Qianyou 3001 100 100 BOA 40 40 40 30 40 40 40 DMAA 5 5 5 ACMO 5 5 5 5 THFA 5 10 5 TS720 20 25 20 15 R972 20 15 15 15 Darocure1 173# 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 TPO-L 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 KBM403 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 KBM5103 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Shear strength(MPa) PE / PE 1.0 1.5 1.2 1.5 0.4 0.3 0.4 0.5 PP / PP 17 2.7 2.9 2.6 1.1 0.5 0.9 1.0 PET / PET 2.0 3.4 3.8 3.5 1.0 2.5 2.7 2.6 POE / POE 3.1 4.2 4.3 4.4 0.9 0.9 1.1 1.2

[0055] Note: TS720 is Cabot hydrophobic silica from the United States; R972 is Evonik hydrophobic silica from Germany; KBM403 is Shin-Etsu γ-glycidyloxypropyl trimethoxysilane from Japan; KBM5103 is Shin-Etsu 3-(acryloyloxy)propyl trimethoxysilane from Japan; 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; PE is polyethylene; PP is polypropylene; PET is polyethylene terephthalate; POE is polyolefin elastomer.

[0056] Preparation of shear strength test piece: Wipe the surface of the test piece with alcohol to remove the oil and dust on the surface. Fix it with a butterfly clip after gluing, and then cure it with ultraviolet light. The ultraviolet light dose is 1000mJ / cm on both sides. 2 , place at room temperature for 30 minutes to test the shear strength. The test method is in accordance with GB / T 7124-2008.

[0057] According to Table 1, by comparing Examples 1 to 4, it is found that the light-curing silicone UV adhesive compounded with acrylate monomers of the present invention shows good bonding strength for PE, PP, PET, POE, etc. However, Comparative Example 1 uses a single light-curing polydimethylsiloxane without compounding with acrylate monomers, and its bonding strength is relatively low. Comparative Examples 2 to 4 are UV adhesives compounded with polyurethane acrylate oligomers available on the market that have good bonding strength to PET films. Although they have good adhesion to PET films, their bonding strength to low surface energy materials such as PE, PP, POE, etc. is very low and does not meet the requirements of practical application.

Claims

1. A photocurable silicone, characterized in that: It is an acrylic modified polydimethylsiloxane with the following structure: Wherein n=an integer from 40 to 2000.

2. A method for synthesizing a photocurable silicone as claimed in claim 1, characterized in that: include: (1) a step of performing a hydrosilylation reaction with a hydrogen-containing double-capping agent and allyl glycidyl ether in the presence of a platinum catalyst to generate a double-capping agent having an epoxy group at each end, (2) a step of subjecting a double-capping agent having an epoxy group at each end and an excess of acrylic acid to a ring-opening esterification reaction in the presence of a ring-opening esterification catalyst to obtain a double-capping agent having an acryloyloxy group and a hydroxyl group at each end, (3) subjecting a double-capping agent having an acryloyloxy group and a hydroxyl group at each end to an esterification and dehydration reaction with excess acrylic acid in the presence of a cyclohexane water-carrying agent and an acidic catalyst to generate a double-capping agent having two acryloyloxy groups at each end, (4) a step of rearranging a double-capping agent having two acryloyloxy groups at each end with polydimethylsiloxane in the presence of a linear chlorinated phosphine catalyst to obtain a high molecular weight acrylic acid-modified polydimethylsiloxane, i.e., a photocurable silicone.

3. The method according to claim 2, characterized in that In both the ring-opening esterification and the esterification dehydration reaction, a free radical inhibitor is added, and the amount added is 0.01% to 1% of the total mass of the reactants.

4. The method according to claim 2, wherein the ring-opening esterification catalyst is selected from tertiary amines and quaternary ammonium salts, and is commonly used any one of triethylamine, N,N-dimethylbenzylamine, N,N-dimethylaniline, trimethylbenzylammonium chloride, triphenylphosphine, triphenylantimony, chromium acetylacetonate, chromium isooctanoate, stannous octoate, and tetraethylammonium bromide, and its usage is 0.1% to 5.0% of the reactants.

5. The method according to claim 2, characterized in that The acidic catalyst is selected from any one of concentrated sulfuric acid, sodium bisulfate, methanesulfonic acid, toluenesulfonic acid, trifluoromethanesulfonic acid, strongly acidic ion exchange resin and solid super acid, preferably any one of methanesulfonic acid, trifluoromethanesulfonic acid, strongly acidic ion exchange resin and solid super acid, and its addition amount is 0.1% to 5.0% of the total mass of the reactants.

6. The method according to claim 2, characterized in that The amount of linear phosphonium chloride catalyst added is 0.005-1.0% of the total amount of reactants.

7. The method according to claim 2, characterized in that After the rearrangement reaction is completed, a passivating agent is used to inactivate the linear phosphonium chloride, and the amount of the passivating agent added is 1.1 to 4.0 times the amount of the linear phosphonium chloride added.

8. A UV adhesive composition for difficult-to-bond materials, characterized in that: The invention comprises, by weight, 100 parts of the photocurable silicone as claimed in claim 1, 10 to 80 parts of acrylate monomers and 0.5 to 5.0 parts of free radical photoinitiator.

9. The UV adhesive composition according to claim 8, characterized in that: The acrylate monomer is selected from any one or two of isobornyl acrylate (IBOA), cyclohexyl acrylate, tetrahydrofuranyl acrylate (THFA), tricyclodecyl acrylate, dicyclopentadiene acrylate, acryloyl morpholine (ACMO) and N,N-dimethylacrylamide (DMAA), preferably isobornyl acrylate, tetrahydrofuranyl acrylate (THFA), acryloyl morpholine (ACMO) and N,N-dimethylacrylamide (DMAA).

10. The UV adhesive composition according to claim 8, characterized in that: The free radical photoinitiator is selected from one or more of 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) and α-aminoalkylphenone.

Images