Study on the Preparation and Performance of Epoxy Emulsion

introduction

Traditional epoxy resins are difficult to dissolve in water, but are soluble in solvents such as aromatic hydrocarbons, ketones, and alcohols

Due to its excellent anti-corrosion and chemical resistance properties, two-component epoxy resin coatings have a wide range of applications

Experimental part

Synthesis of emulsifier

Add a certain amount of PEG-4000 and phthalic anhydride into a four-necked flask with reflux condenser, mechanical stirrer, constant pressure dropping funnel and thermometer, raise the temperature and start stirring to make the raw materials evenly mixed

The synthesis reaction of the emulsifier is divided into two steps: the first step is the esterification reaction, that is, the half-esterification of the hydroxyl group of polyethylene glycol and the acid anhydride to produce intermediate product A; the second step is the ring-opening reaction, that is, the product A The ring-opening addition of the remaining carboxyl groups and epoxy groups generates the final target product emulsifier

Preparation of epoxy emulsion

Heat and melt the metered solid E20 and the above emulsifier, the temperature is controlled at about 80℃; after the melting is complete, the rotation speed is 1400~1600r/min for physical dispersion and mixing, the dispersion time is about 20min; the deionized water is preheated to 80℃, after the resin and emulsifier are completely melted, slowly add deionized water dropwise, and stop the dripping until the reverse rotation stage, and disperse for 90 minutes.

Preparation of water-based two-component epoxy coating

According to the formula, the pigments, fillers, additives and deionized water are grinded and stirred at high speed to form a color paste, and then the prepared color paste is added to the self-made epoxy emulsion to disperse at high speed to form the first component, and finally the first component and the curing agent After mixing and mixing, the plate is made to prepare for testing performance

Results and discussion

(1) The influence of the ratio of raw materials on the conversion rate of synthetic emulsifier

When the emulsifier is synthesized, the temperature is set to 110~120℃, the esterification reaction time is 2.

When the amount of PEG-4000 added is too much and there is more E51 in the system, the product will appear gel
.
The main reason is that too many hydroxyl groups and phthalic anhydride are left after the reaction is completed.
The remaining hydroxyl groups directly undergo ring-opening reaction with epoxy groups under the action of the catalyst, and the reaction is relatively violent, which eventually leads to gelation
.
In summary, when the experiment determines the synthesis reaction of the emulsifier, n(PEG-4000):n(phthalic anhydride):n(E51)=1.
2:2:1.
8 is more appropriate
.

(2) The influence of reaction temperature on the conversion rate of synthetic emulsifier

Determine n(PEG-4000):n(phthalic anhydride): n(E51)=1.
2:2:1.
8, the esterification reaction time is 2.
5~3h, the ring-opening reaction time is 3~4h, and the reaction temperature during the synthesis of the emulsifier is investigated Impact on conversion rate
.
The results show that when the synthesis reaction temperature is less than 110°C, the conversion rate of the reactive groups in the material is low, and as the reaction temperature increases, the conversion rate also increases
.
When the temperature exceeds 120°C, the conversion rate cannot be increased even if the temperature is increased, and excessively high temperature may cause side reactions or the occurrence of trace gels, which will affect the conversion rate instead
.
Therefore, it is determined that the reaction temperature during the synthesis of emulsifier is between 110 and 120 ℃
.

(3) The influence of reaction time on the conversion rate of synthetic emulsifier

Determine n(PEG-4000):n(phthalic anhydride): n(E51)=1.
2:2:1.
8, the reaction temperature is 110~120℃, and the time of the two-step reaction of esterification and ring opening when synthesizing emulsifier is investigated.
The impact of conversion rate
.
The results showed that with the progress of the reaction, the first stage of the esterification reaction became stable after 2.
5h, and its conversion rate hardly changed after 3h, so the time for the esterification reaction was determined to be 2.
5~3h
.
The ring-opening reaction in the second stage tends to be stable after 3 hours.
The reaction continues slowly, but it is also approaching the end.
The increase in conversion rate is small and does not change after 4 hours.
Therefore, the time for the ring-opening reaction is controlled at 3.
~4h is more appropriate
.

The time of the esterification reaction is about 0.
5~1h less than the time of the ring-opening reaction.
Analysis of the reason may be because the esterification reaction is an anhydride-opening half-esterification reaction of hydroxyl and acid anhydride.
The activation energy required for this reaction is relatively low, so Relatively easy to proceed; and the ring-opening reaction is the addition reaction of epoxy groups and carboxyl groups.
Although a catalyst is added, the reaction requires a higher activation energy because the reaction is not easy to proceed, so it is slightly more difficult than the esterification reaction.
Some
.

(4) Performance analysis of epoxy emulsion

❖The effect of the amount of emulsifier on the performance of epoxy emulsion

In the preparation of epoxy emulsion, the stability of the epoxy emulsion and the water resistance of the coating were investigated by controlling the different addition amount of the emulsifier.
The results showed that the stability of the epoxy emulsion continued to improve with the increase of the amount of emulsifier
.
However, the water resistance of the coating film increased first and then decreased
.
The reason may be that when the amount of emulsifier is small, on the one hand, the amount of emulsifier is not enough to completely cover the water droplets in the system, resulting in a decrease in the strength between particle interfaces and weakening of the repulsive force between water particles; on the other hand, Under the action of shearing force, the unwrapped water particles form larger water droplets due to the association of hydrogen bonds, which, as a continuous phase, reversely reverse in advance during the dispersion process
.
The water particles in the dispersed phase are fixed in the coating of the emulsifier because it is too late to combine with the water droplets in the continuous phase, so the whole system forms a water/oil/water structure, and the water coated in the emulsifier The larger particles cause the stability of the epoxy emulsion to decrease
.

With the increase of the amount of emulsifier, the emulsion micelles formed in the system are more uniform, so that the emulsion can be smoothly changed from water/oil to oil/water during preparation, and the stability of the emulsion increases accordingly
.
Similarly, when the amount of emulsifier is small, the reverse rotation process cannot be completed during emulsification.
The resin that has not reversed in the emulsion causes the particle size to become larger, and the dense cross-linked structure cannot be formed during the curing process with the curing agent.
Water resistance is poor; as the amount of emulsifier increases, the balance between epoxy resin and water can be reached at a certain amount, so that the stability of the emulsion and the water resistance of the coating film are better; but because the emulsifier is two The hydrophilic substance has good hydrophilicity.
As its dosage increases, the water resistance of the final coating film will decrease
.
Experiments show that the stability and water resistance reach the optimal balance when the amount of emulsifier is about 15%
.

❖The effect of dropping temperature on the stability of epoxy emulsion

During the preparation of the emulsion, in the reverse procedure, the stability of the final emulsion was investigated by controlling different dripping temperatures.
The results showed that as the dripping temperature increased, the stability of the final emulsion also increased and then decreased.

.
Analysis of the reason is that the viscosity of the epoxy resin after dissolution decreases due to the increase of temperature, and it can be fully dispersed with the emulsifier; on the other hand, the activity of the emulsifier itself will increase with the increase of temperature, the effect of both The stability of the emulsion is improved after the dropping temperature is increased
.
However, when the dropping temperature is too high, the bond energy (mainly hydrogen bonds) formed between the emulsifier and water will be weakened, and the emulsifier and water cannot form a uniform and stable system, resulting in poor emulsion stability
.
This experiment shows that the stability of the final emulsion is better when the dripping temperature is 80°C when reversed
.

❖The influence of the speed and dispersion time of the reverse rotation phase on the stability of epoxy emulsion

In the phase of reverse rotation, by controlling different rotation speed and dispersion time, the effect on the stability of the final emulsion was investigated.
The results showed that with the increase of rotation speed, the stability of the emulsion also increased, but the stability when the rotation speed was too high It will drop again
.
The main reason is that when the rotation speed is increased, the large water droplets that are not easy to be sheared become small water droplets, which are more easily covered by the emulsifier, thereby improving the stability of the final emulsion; but the rotation speed is too fast to easily damage the interface.
The force between them makes the particles of the same item agglomerate into agglomerates, which will also reduce the stability of the emulsion.
In summary, the experiment has determined that the stability of the emulsion is the best when the rotation speed is about 2000 r/min
.
With the extension of the dispersion time in the reverse phase, the stability of the emulsion becomes more and more stable.
Considering the production efficiency, the experiment determined that the dispersion time of the reverse phase is about 90 minutes
.

(5) Analysis of coating film performance of epoxy emulsion

In the experiment, the company’s product 740B was selected as the water-based epoxy curing agent, and the performance of the coating film prepared by a competitor epoxy emulsion and a self-made epoxy emulsion in the industry was compared.
, Hardness and gloss are better than competing emulsions.
Analysis of the reasons is mainly because the structure of the benzene ring and long-chain polyols in the emulsifier improves the hardness, dryness and gloss.
In addition, in the preparation of epoxy emulsions In the process, the appropriate amount of emulsifier and preparation temperature are greatly helpful to the water resistance and dryness of the final coating film; finally, the epoxy resin selected when preparing the epoxy emulsion has a great effect on the gloss, water resistance and hardness of the final coating film.
It also has a certain impact
.

The salt spray test results show that after 500 hours, the coating film made by the competing emulsion has fallen off, and the coating film made by the self-made epoxy emulsion has no blistering, severe loss of gloss or even paint film except for a small amount of rust.
In the case of shedding, there is no obvious corrosion phenomenon, which shows that the self-made epoxy emulsion is better than the competing epoxy emulsion in terms of salt spray resistance
.

Concluding remarks

The experiment is controlled by n(PEG-4000):n(phthalic anhydride):n(E51)=1.
2:2:1.
8, the esterification reaction time is 2.
5~3h, the ring-opening reaction time is 3~4h, and the reaction temperature is 110~120 Under the condition of ℃, an epoxy emulsifier was prepared
.
Through a certain amount of E20 epoxy resin and 15% of the epoxy emulsifier prepared above, an epoxy emulsion is prepared
.

The two-component epoxy coating prepared by the epoxy emulsion and waterborne epoxy curing agent prepared in this experiment can form a better film, and compared with the competition epoxy emulsion in the same industry, the coating film has better drying properties , Hardness, gloss, water resistance and excellent salt spray resistance, can be widely used in various light, medium and heavy industrial anti-corrosion fields
.

References (omitted)

Authors | Dai Guoxu, Liu Fengguang

(Changzhou Guangshu Chemical Technology Co.
, Ltd.
)