Preparation of low viscosity epoxy acrylic acid photopolymer prepolymer in light curing system

With the integration and development of materials engineering, applied mechanics, automatic control and bionics, light cured composite has become one of the most favourite research topics in the field of materials and engineering at home and abroad. In the UV curing system, the prepolymer and the reactive diluent form the backbone of the cured material together. And they account for more than 90% of the total mass. The basic properties of the cured product are mainly determined by the prepolymer. A low viscosity epoxy acrylate photosensitive prepolymer with a viscosity of 6800 mPa • s (25 °C ) was obtained by esterification of 5 hours with bisphenol A epoxy resin with high epoxy value and low viscosity.


Introduction
The photocurable material is prepared on the basis of prepolymer, adding specific reactive diluents, photoinitiator and various additives. Under the irradiation of the curing light source, the photoinitiator in the liquid light-curing material is excited to become free radical or cation, which causes the material containing unsaturated double bond in the material to form a solidification reaction.
In the UV curing system, the prepolymer and the reactive diluent form the backbone of the cured material together. They account for more than 90% of the total mass [1]. The basic properties of the cured product are mainly determined by the prepolymer. According to the mechanism of light curing, the suitable prepolymer is also different [2].
(2) Cationic polymerization mechanism The applicable prepolymer mainly includes epoxy resin, epoxy functionalized polysiloxane resin, and resin containing vinyl ether functional group.
Modern industrialized acrylated prepolymers are predominantly present: acrylated epoxy resins, acrylated urethanes, acrylated polyesters, and acrylated polyacrylates. And the former two are the most important. Table 1 lists the performance of common prepolymers [3].  The selection of prepolymer in light curing system mainly takes the following factors into consideration: low viscosity, light curing speed, good physical and mechanical properties, glass transition temperature, curing shrinkage, low toxicity and low irritation [4].
Acrylic series resin is the second generation resin of light curing system. It is introduced into the functional group of the crosslinking reaction at the end of some prepolymer by introducing double bond into the acrylate [5]. Common polyester acrylate, polyether acrylate, epoxy acrylate, polyurethane acrylate, melamine acrylate. Among them, epoxy acrylate (EA) attracted much attention because of easy synthesis, strong adhesion, chemical resistance, high strength characteristics [6].
Epoxy acrylate can be divided into bisphenol A epoxy acrylate resin, novolac epoxy acrylate, epoxy acrylate and epoxy acrylate oil according to the structure. Among them, the bisphenol A type epoxy acrylate is the most stable [7]. Bisphenol A epoxy acrylate is obtained by reacting bisphenol A epoxy resin with (meth) acrylic acid [8]. Bisphenol A epoxy acrylate was prepared from resin and acrylic acid as prepolymer of light repairing agent [9].

Reaction mechanism
Epoxy acrylates are formed by the reaction of epoxy resin and acrylic acid,and the preparation conditions vary depending on the type of EA. Catalysts that promote the reaction of epoxy groups and hydroxyl groups include strong bases and organic amines. The reaction between the epoxy group and the hydroxyl group is an exothermic reaction. It is very important to select a suitable polymerization inhibitor to prevent radical polymerization of unsaturated groups on acrylic acid during the preparation of epoxy acrylates. The common inhibitors are phenols (such as hydroquinone, etc.). The synthesis of epoxy acrylate is as follows [10].

Main reaction.
Where R is: 2nd International Conference on New Material and Chemical Industry (NMCI2017) IOP Publishing IOP Conf. Series: Materials Science and Engineering 292 (2018) 012108 doi:10.1088/1757-899X/292/1/012108 2.1.2. Side reaction. The side reactions mainly include: The self -polymerization reaction occurs after epoxy ring -opening and reaction of epoxy groups with active hydrogen to form hydroxyl groups.

Evaluation of prepolymer quality
Measures to measure the quality of epoxy acrylate mainly consist of acidity value and viscosity [11]. In the reaction, the acidity value indicates the degree to which the reaction (esterification) proceeds (the conversion of the esterification reaction). The viscosity indicates the molecular size of the resin. In the experiment, a reasonable preparation process was selected by comparing the acidity value and viscosity of different reaction times [12].

Determination of acidity value.
Weigh the quantitative samples dissolved in alcohol, adding phenolphthalein, titration with standard solution of potassium hydroxide; calculate the acidity value of the epoxy acrylic resin according to the following formula.

AV--acidity value
V--the volume of KOH solution is consumed C--concentration of KOH solution m--sample weight The acidity value before reaction can be calculated according to formula (2) AV 0 --initial acidity value G--the total mass of the reactants K--molar number of acids The reaction conversion P:

Determination of viscosity.
The reaction product (prepolymer) of epoxy resin with acrylic acid is Newton fluid. There are many methods for measuring viscosity of Newton fluid, such as capillary method, rotation method, etc. This experiment uses falling-ball method [13].

Experimental equipment
Experimental equipment is shown in figure 1. It includes: thermostat water bath, mixer, three-necked flask, thermometer, dropper, and so on.

Experimental method
(1) Preparation of phenolphthalein indicator and concentration of 0.1mol / L KOH-ethanol solution.
(2) Heat the thermostat water bath at a temperature of 90℃. Take proper amount of E-51, put it into three-necked flask, and heat three-necked flask in the thermostat water bath.
(3) Add acrylic acid to the three-necked flask slowly and stir constantly during the drip process. After completion of the drop in the corresponding time, heat the thermostat water bath into 100℃ and continue stirring.
(4) Measure the acidity value of the reactants in the three-necked flask at intervals of a certain time.
(5) After two hours of reaction, take a certain amount of reactants at intervals of one hour and measure the viscosity of the reactants using falling-ball method.

Acidity value and conversion rate in the reaction process
(1) According to formula (2), the initial acidity value before reaction is 155.6mg/g.
(2) In the process of reaction, the amount of acid was measured at regular time, and the acidity value and conversion rate were calculated according to formula (1) and formula (3). Table 2 shows the change of acidity value during the reaction. Figure 2 and figure 3 show the acidity value and conversion rate of reactants in different reaction stages.  Figure 2. Acidity value-Reaction time. Figure 3. Conversion rate-Reaction time.

Experimental results
Experimental results show that: (1) The initial esterification speed of epoxy resin and acrylic acid is faster. After 1 hour esterification, the conversion rate can reach 53.2%, and the corresponding acidity value is reduced to 72.8 mg/g.
(2) With the prolonging of esterification time, the conversion rate and acidity value gradually tend to be stable. When the reaction proceeds to 5 hours, the acidity value reaches a steady value of 5.6 mg/g. And the conversion rate of esterification is over 95%. As the reaction time increases, the conversion and the change of acidity value is no longer obvious. It indicates that the esterification reaction is basically over.
(3) As shown in figure 4, the viscosity of the reaction product of epoxy resin and acrylic acid increase with the progress of the reaction. The increase in viscosity is due to the fact that the macromolecule product is produced with the esterification reaction.
(4) As shown in figure 5, the viscosity of the reactants decreases with increasing temperature and the difference in viscosity of the reactants in the different esterification stages decreases as the temperature increases. When the temperature is 25 ℃ ~ 35 ℃, the viscosity of each reactant is relatively stable. Thus, the use of a viscosity of 25 ℃ to characterize the reactant viscosity is comparable.
(5) When the esterification reaction was carried out for 5 hours, the acidity value and the conversion rate of the reactants had reached a stable value. At this point, the viscosity of the reactant is 6800m Pa•s(25℃).

Conclusion
(1) The reaction temperature was divided into two stages (low temperature dropping and constant temperature reaction). It can effectively prevent the self-polymerization of acrylic acid and gel of hydroxyl groups of epoxy resin and acrylic acid. It is beneficial to reduce the viscosity of prepolymer and guarantee its quality by dividing the reaction temperature into two stages (low temperature dropping and constant temperature reaction) and strictly controlling the reaction temperature and reaction time.
(2) A low viscosity epoxy acrylate photosensitive prepolymer with a viscosity of 6800 mPa•s(25℃) was obtained by esterification of 5 hours with bisphenol A epoxy resin with high epoxy value and low viscosity. Compared with the viscosity value of partially low viscosity epoxy acrylic acid, the viscosity of epoxy acrylic acid prepared in this study is greatly reduced, which can meet the requirement of viscosity of light-repair. Table 4 is the viscosity comparison of the different prepolymers [1].