Synthesis of 1,1,3,3,5,5-hexamethyl-1,5-bis[(3-ethyl-3-methoxyoxetane)propyl]trisiloxane and Research on Its UV-curing Performance

HU Siwei, HUANG Biwu, CHEN Weiqing

(School of Materials Science and Engineering, Nanchang University, Nanchang 330031, China)

Abstract: A compound, 3-ethyl-3-hydroxymethyloxetane (EHO), was synthesized with diethyl carbonate and trihydroxypropane as raw materials, 3-ethyl-3-allylmethoxy oxetane (EAMO) was synthesized with EHO and allyl bromide, and 1,1,3,3,5,5-hexamethyl-1,5-bis[(3-ethyl-3-methoxyoxetane)propyl]trisiloxane (HMBEMOPTS) was synthesized with EAMO and 1,1,3,3,5,5-hexamethyltrisiloxane (HMTS). HMBEMOPTS is a novel UV-curable oligomer. The test of photo-DSC shows the photosensitivity of HMBEMOPTS is better than the ordinary oxetane, 3-ethyl-3-[(3-ethyloxetan-3-yl)methoxymethyl]oxetane. HMBEMOPTS was mixed with bisphenol A type epoxy resin E-51 to prepare a cationic UV-curable system, and triarylsulfonium hexafluoroantimonate (UV-6976) was used as a cationic photoinitiator. The mechanical tests of coating films prove that when the mass fraction of HMBEMOPTS is 50%, the mechanical properties of the curing system are the best. The impact strength of the UV-curable films is measured to be 40 kg·cm and the flexibility is 2 mm; the tensile strength and flexural strength of the prepared specimens are 20.74 MPa and 13.43 MPa, respectively. The experimental results show that HMBEMOPTS can effectively improve photosensitivity and flexibility of the photosensitive resin.

Key words: oxetane; siloxane; photosensitive resin

1 Introduction

Ultraviolet (UV) curing technology has become a hot spot for researchers in the past twenty years due to its instant curing, high efficiency, environmental friendliness, and unique advantages of adjustable curing coating properties (from brittle to very soft films)[1-4]. Comparing with traditional coatings, UV-curable coatings can save energy and reduce the content of volatile organic compounds[5,6]. As a typical thermosetting material, UV-curing material has many advantages, such as excellent mechanical properties, heat resistance, load carrying capacity, and dimensional stability, so it is widely used in many fields, including coatings, electronic packaging materials, adhesives and composite materials[7-9].

According to different curing mechanisms, UV-curing can be divided into free radical curing and cationic curing. The main advantage of free radical curing is the fast curing speed[10]. But the reaction is easy be inhibited by oxygen in the air[11], and a significant volume shrinkage will occur during the curing process[7]; the characteristics of cationic curing are completely different, the polymerization will not be affected by oxygen, the shrinkage rate generated during curing is small, and the adhesion of the cured sample is also excellent[12]. Duo to these characteristics, cationic polymerization also serves as a key technological platform in many imaging applications, such as photolithography, stereolithography, and additive manufacturing[13-15].

However, the main reasons that limit the application of cationic polymerization in the field of UV-curing include low reactivity of monomer and few monomers that can be used for curing. Monomers that can be polymerized by cationic mechanism mainly includes epoxy, oxetane, vinyl ether and their derivatives[11]. Epoxides applied in cationic photosensitive resins include cycloaliphatic and aliphatic epoxy resin[16]. The UV-curing rate of cycloaliphatic epoxide is fast but the cured resin is very brittle[17]. Aliphatic epoxide has better flexibility due to the flexible molecular structure but the reactivity is low[18]. Although vinyl ether has high reactivity[19], it also has the shortcomings of volatility and environmental pollution. Therefore, many scholars have tried to develop a novel cationic photosensitive resin with high photosensitivity and low viscosity.

In this article, a novel oxetane EAMO was synthesized with diethyl carbonate, trihydroxypropane and allyl bromide as raw materials. Oxetane is a kind of strain and reactive heterocyclic compound containing four-membered cyclic in its structure, it can undergo cationic ring-opening polymerization and has many excellent properties including high adhesion, low viscosity and curing shrinkage[20,21]. But the photosensitivity of commercially available oxetane is not very good, so it is difficult to apply to the occasions that require high photosensitivity, such as stereolithography. In order to further improve the photosensitivity of oxetane derivatives, we used HMTS which is an important inorganic material with low surface energy and good film-forming properties[22]to copolymerized with EAMO to obtain a novel UV-curable material HMBEMOPTS. The test of photo-DSC shows that the photosensitivity of the synthesized product is better than the existing oxetane. Then HMBEMOPTS was blended with E-51 to prepare a cationic UV-curing system, and the mechanical properties of the samples were tested.

2 Experimental

2.1 Raw materials

Diethyl carbonate, trihydroxypropane, allyl bromide, potassium hydroxide, tetrabutylammonium bromide, hexamethyltrisiloxane, caster catalyst, dichloromethane, anhydrous magnesium sulfate, triarylsulfonium Onium hexafluoroantimonate (cationic photoinitiator, UV-6976) and 3-ethyl-3-[(3-ethyloxetan-3-yl)methoxymethyl]oxetane are analytically pure; E-51 is industrial product.

2.2 Instruments

The UV-curing process was carried out in an INTELLI-RAY400 intelligent control light curing system (Huishuo Electromechanical Co., Shenzhen, China). The infrared spectrum of the synthesized photosensitive resin was characterized by the Nicolet5700 intelligent fourier transform infrared spectrometer made in the United States. The NMR spectrum of the synthesized photosensitive resin was characterized by a 400MR DD2 nuclear magnetic resonance spectrometer made in the United States. The photosensitivity of the samples was tested by TA Instruments Q2000 Photo-DSC apparatus equipped with mercury lamp and UV-intensity of 35 mW/cm2. The flexibility and impact properties of the UV-cured films were tested by the QTX type paint film flexibility tester and QXJ type paint film impact tester (Yonglida Material Testing Machine Co., Tianjin, China). The mechanical properties of the cured samples were tested with the CMT4204 microcomputer-controlled universal material testing machine (New Sansi Company, Shenzhen, China).

2.3 Synthesis of photosensitive resin

2.3.1 Synthesis of EHO

First, trihydroxypropane and diethyl carbonate were used for transesterification reaction in alkaline environment to generate six-membered ring lactone intermediates, then they were pyrolysised to remove CO2and collected 130-135 ℃ fractions after vacuum distillation, the precursor EHO was obtained, and the synthesis route is shown in Scheme 1.

Scheme 1 Synthetic method for EHO

Scheme 2 Synthetic method for EAMO

Scheme 3 Synthetic method for HMBEMOPTS

2.3.2 Synthesis of EAMO

The EHO prepared by the reaction and aqueous potassium hydroxide solution were added to a threenecked round bottom flask equipped with a stirrer, a thermometer, a condenser and a dropping funnel. We added tetrabutylammonium bromide under stirring, then controlled the droplet speed of the dropping funnel to add allyl bromide dropwise and kept it at 0 ℃ for 24 h; when the reaction was complete, a certain amount of dichloromethane and distilled water were added to the reaction product and a separatory funnel was used for phase separation. The separated organic phase was washed twice with distilled water and dried over anhydrous magnesium sulfate, then the organic phase was vacuum distilled to remove excess allyl bromide and dichloromethane. Finally, the remaining fraction was vacuum distilled to collect the fraction at 55-60 ℃, the synthesis route is shown in Scheme

2.2.3.3 Synthesis of HMBEMOPTS

EAMO and caster catalyst were added to a threenecked round bottom flask equipped with a stirrer, a thermometer, a condenser and a dropping funnel, then we added appropriate amount of toluene. When the temperature of the reactants in the three-neck flask rose to 80 ℃, the mixture of toluene and HMTS was added to the flask slowly and kept the reaction temperature at 88-90 ℃. After the addition was completed, the mixture was kept warm and stirred for 8 hours. After that, the solvent and excess reactants were removed by a rotary evaporator at 95 ℃ under vacuum. The synthetic route is shown in Scheme 3.

3 Results and discussion

3.1 Characterization of EHO

Fig.1 is the FT-IR spectrum of EHO, the absorption peak at 3 410 cm-1corresponds to the absorption peak of hydroxyl groups; the absorption peak of C-H bonds in the methyl groups shows at 2 960, 2 880, 1 480 cm-1; the absorption peak at 2 934 cm-1corresponds to the C-H bonds in the methylene groups; the absorption peak at 823 cm-1is the symmetrical deformation vibration absorption peak of the four-membered oxetane rings[23]; the absorption peak at 974 cm-1is due to the antisymmetrical stretching of C-O-C in the oxetane rings.

Fig.1 FT-IR spectrum of EHO

Fig.2 is the1H-NMR spectrum of EHO when CDCl3was used as the deuterated solvent. It can be seen from the figure that the proton nuclear magnetic resonance spectrum of the product has 6 peaks. The specific analysis is shown in the Table 1.

Combining Table 1 and Fig.2, the peak at 3.74 ppm corresponds to the vibration of the hydroxyl groups. The peak area ratio of the 6 peaks is 3:2:2:1:2:2. It is in line with the number and number ratio of hydrogen atoms in the EHO molecular chain structure. These can prove the product obtained is EHO.

Fig.2 1H-NMR spectrum of EHO

Table 1 Analytical table of the 1H-NMR spectrum for the EHO

3.2 Characterization of EAMO

Fig.3 shows the FT-IR spectra comparison of synthesized EAMO and EHO. It can be clearly observed that the absorption peak of the hydroxyl groups at 3 410 cm-1disappeared, while the characteristic absorption peaks of allyl groups appear at 3 080, 1 645, and 925 cm-1. It shows that EHO has reacted completely, and the product obtained by the reaction is EAMO.

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Fig.3 FT-IR spectra of EAMO and EHO

Fig.4 is the1H-NMR spectrum of EAMO when CDCl3was used as the deuterated solvent. It can be seen from the figure that the proton nuclear magnetic resonance spectrum of the product has 8 peaks. The specific analysis is shown in the Table 2.

Combining Table 2 and Fig.4, the peaks at 5.13-5.30 ppm and 5.82-5.94 ppm are correspond to the vibration of the C=C. There is no absorption peak of hydroxyl groups at 3.74 ppm. And the peak area ratio of the 8 peaks is 3:2:2:2:2:2:2:1. It is in line with the number and number ratio of hydrogen atoms in the EAMO molecular chain structure. These can prove the product obtained is EAMO.

Table 2 Analytical table of the 1H-NMR spectrum for the EAMO

Fig.4 1H-NMR spectrum of EAMO

3.3 Characterization of HMBEMOPTS

Fig.5 is the FT-IR absorption spectra of HMBEMOPTS, HMTS, and EAMO. In the curve of HMBEMOPTS, the strong peaks at 797 cm-1and 840 cm-1correspond to the stretching vibration of the Si-C bonds and the in-plane swing of methyl, the strong and broad peak at 1 050 cm-1corresponds to the stretching vibration of the Si-O-Si. The absorption peaks at 1 258 and 1 410 cm-1correspond to the in-plane and out-of-plane flexural vibration of the methyl. It can be seen that the flexural vibration absorption peaks of the Si-H bonds at 2 126 and 909 cm-1are weak, while the characteristic absorption peaks of allyl groups do not appear at 3 080, 1 645 and 926 cm-1, indicating that the silicon-hydrogen addition reaction is complete.

Fig.5 FT-IR spectra of HMBEMOPTS, HMTS, and EAMO

Fig.6 is the1H-NMR spectrum of HMBEMOPTS when CDCl3was used as the deuterated solvent spectrum. It can be seen from the figure that the proton nuclear magnetic resonance spectrum of the product has 10 peaks. The specific analysis is shown in the Table 3.

Fig.6 1H-NMR spectrum of HMBEMOPTS

Table 3 Analytical table of the 1H-NMR spectrum for the HMBEMOPTS

Combining Table 3 and Fig.6, the peak of Si-H at 4.66-4.73 ppm, the peaks of C=C at 5.13-5.30 ppm and 5.82-5.94 ppm do not appear in the1H-NMR spectrum of the product, it can prove that the silicon-hydrogen addition reaction is complete. The peak area ratio of the 10 peaks is 3:6:2:3:2:2:2:2:2:2. It is in line with the number and number ratio of hydrogen atoms in the molecular chain structure of the synthesized product. These can prove that the final product is HMBEMOPTS.

3.4 Performance test of cationic photosensitive resin

3.4.1 The photosensitivity test of siloxane-modified oxetane

Figs.7-9 are the p-DSC thermograms of HMBEMOPTS, 3-ethyl-3-[(3-ethyloxetan-3-yl)methoxymethyl]oxetane and E-51, respectively. Photoelectric Differential Scanning Calorimetry (p-DSC) is used to measure the heat flux of the exothermic reaction caused by light over time at a given temperature and has become an important technology that can monitor the kinetics of the UV-curing process[24].

It can be seen from the comparison of the three figures that the heat flux peak of E-51 appears at about 110 second, while the heat flux peaks of oxetanes appear much earlier than it. This shows that among the three samples, E-51 has the worst photosensitivity, it also proves that oxetane and its derivatives have good photosensitivity and can be used to make up the shortcoming of the poor photosensitivity of E-51.

Comparing Fig.7 with Fig.8, it can be seen that the heat flow peak in Fig.7 appears earlier. And in Fig.7, the heat flow generated by the curing reaction of HMBEMOPTS is relatively low at 40 second, indicating that the degree of reaction is high; but in Fig.8, the curing reaction still produces a relatively high heat flow after 40 second, and the reaction continues to react for nearly 60 seconds at that speed. This means that the curing rate of 3-ethyl-3-[(3-ethyloxetan-3-yl)methoxymethyl]oxetane is much slower than HMBEMOPTS. The main reason for the differences in photosensitivity between HMBEMOPTS and 3-ethyl-3-[(3-ethyloxetan-3-yl)methoxymethyl]oxetane is that the molecular chain of 3-ethyl-3-[(3-ethyloxetan-3-yl)methoxymethyl]oxetane is composed of C-O bonds instead of Si-O bonds. By comparing Fig.7 with Fig.8, it can prove that the photosensitivity of HMBEMOPTS is much better than ordinary oxetane.

Fig.7 p-DSC thermograms of HMBEMOPTS

Fig.8 p-DSC thermograms of 3-ethyl-3-[(3-ethyloxetan-3-yl)methoxymethyl]oxetane

Fig.9 p-DSC thermograms of E-51

3.4.2 Stability of siloxane-modified oxetane

The synthesized HMBEMOPTS was stored in the dark, and its viscosity at 25 ℃ was measured. The effect of storage time on the viscosity of HMBEMOPTS is shown in Fig.10. As the storage time increases, the viscosity of HMBEMOPTS will increase. But after 6 months of storage, the viscosity of the samples only increased from 13 mPa·s to 27 mPa·s, which proves that the stability of the synthesized HMBEMOPTS is pretty good.

Fig.10 The effect of storage time on the viscosity of HMBEMOPTS

3.4.3 The effect of siloxane-modified oxetane on the properties of cationic photosensitive resin coating

HMBEMOPTS and E-51 were blended in different proportions, then cationic photoinitiator UV-6976 was added to the mixture, the mass fraction of UV-6976 was 3% of the resin system. According to the GB/T 1731-79 standard, photosensitive resin was apply to a special iron sheet with a smooth surface and placed in a UV-curing machine, the curing parameters were set to 100% light intensity for 120 seconds, and the cured samples were tested for flexibility and impact strength, the experimental results are shown in the Table 4.

Table 4 The effect of different proportions of resin on the properties of cured film

It can be seen from Table 4 when HMBEMOPTS is blended with E-51, it can effectively improve its flexibility and impact strength. As the amount of HMBEMOPTS increases, the flexibility of the cured film increases, but the impact strength first increases and then decreases. This is because HMBEMOPTS has a regular structure and a longer molecular chain. At the same time, the Si-O bonds have good flexibility, which can reduce the frictional resistance between molecular chains and enhance the mobility between molecules. Therefore, with the addition of HMBEMOPTS, the proportion of flexible chains in the resin system continues to increase, and the flexibility of the cured film will continue to increase. Due to the rigidity of the benzene ring in the structure of E-51, the impact resistance of the product obtained directly after curing is very poor, but with the addition of HMBEMOPTS, the crosslinking density and rigidity of the cured film will decrease, so the impact strength will be improved. When the content of HMBEMOPTS reaches a certain level, the cohesive energy of the resin system will decrease and the impact strength of the cured film will decrease.

3.4.4 Mechanical properties of the novel cationic photosensitive resin

HMBEMOPTS, E-51, and UV-6976 were blended to prepare cationic photosensitive resin. The mass fractions of HMBEMOPTS and UV-6976 were 50% and 3% of the cationic UV-curing system, respectively. According to the GB/T 1040.3-2006 and GB/T 9341-2000 standard, the stirred photosensitive resin was poured into the mold to prepare tensile and flexural specimens, and the mold filled with resin was placed into the UV-curing machine with light intensity of 100% and radiation time of 180 seconds. After taking it out, the specimens would be placed in an oven at 120 ℃ for an hour. Finally the mechanical properties of the UV-cured specimens were tested, and the results are shown in the Table 5.

Table 5 Test of the mechanical properties of the prepared specimens

4 Conclusions

In this article, diethyl carbonate, trihydroxypropane, allyl bromide, and HMTS were used to prepare a novel UV-curable material HMBEMOPTS. HMBEMOPTS has excellent photosensitivity which is much better than ordinary oxetane. The viscosity of HMBEMOPTS is only 13 mPa·s at 25 ℃, and its stability is also very good. Then HMBEMOPTS and E-51 were blended in different ratios, and triarylsulfonium hexafluoroantimonate was used as a photoinitiator to prepare a cationic UV-curable resin. The mechanical properties of the resin are pretty good, and when the mass fraction of HMBEMOPTS in the resin is 50%, the mechanical properties of the resin are the best.

The experimental results show that HMBEMOPTS can effectively improve the flexibility and photosensitive of cationic UV-curing resin. It will have a good promotion and application value in the field of UV-curing.