Preparation and Properties of Hydrophobic and Oleophobic Coating for Inkjet Printing

Abstract

As a functional decorative material on the surface of printing and packaging, coating plays the role of increasing gloss, wear resistance, and antifouling. It has broad application prospects in high-end printed materials such as posters, art reproductions, and maps. In this paper, dodecafluoroheptyl methacrylate was used as the fluorine-containing monomer, which was used to modify the epoxy resin and introduce fluoride first. Under the action of polymerization inhibitor and catalyst, the epoxy resin was further modified by ring opening, esterification, and neutralization with acrylic acid, maleic anhydride, and organic base as raw materials. Additionally, a fluorine-containing coating with hydrophobic and oleophobic properties was obtained finally. The effects of fluorine modification of epoxy resin and synthetic polymer were characterized by infrared spectrometer and photoelectron spectroscopy. The results showed that fluorine monomer could be successfully grafted to the molecular body of epoxy resin, and fluorine had been introduced into the surface of the polymer film. Using the contact angle tester, combined with the performance parameters such as grafting rate, thermal stability, adhesion, and gloss, the effects of the amount of fluorine monomer on the properties of the synthetic coating were discussed. The results showed that the hydrophobic and oleophobic properties of the copolymer film surface were closely related to the amount of fluorine monomer. When the molar ratio of the epoxy group to dodecafluoroheptyl methacrylate was 26:1, the mass fraction of fluorine on the film surface was 18.09%, and the contact angles of water and ethylene glycol were 121.8° and 78.2° respectively. At the same time, the printability of self-made hydrophobic and oleophobic coating was tested in this paper. The liquid repellency of inkjet printing before and after glazing and the influence of the coating on the optical properties of printed images were discussed and studied too. The results showed that the coating synthesized by the experiment was suitable for inkjet printing. It had improved the printing quality performance and the functional modification on the surface of inkjet printings, such as liquid repellency, gloss, and color reproduction.

1. Introduction

Digitalization is deeply affecting every industry in the world, and digital reforms are also being carried out in the printing industry [1,2]. Digital printing is different from traditional printing. It does not need film and does not need process operations, which maybe include color separation, plate making, plate revision, plate loading, plate moistening, ink blending, etc. Digital printing is an on-demand, fast, exquisite, and economical printing technology [3,4,5,6,7,8]. As one of the mainstream technologies of digital printing, inkjet printing has the unique advantages of imaging without intermediate media, a wide application range, flexible control, and stable and reliable quality. It is a printing technology without contact with the substrate, pressure, and printing plate [9,10,11]. With the rapid expansion of the demand for on-demand printing of books, maps, and other publications and the requirements of environmental protection, inkjet printing technology, as an efficient and environmentally friendly digital printing technology, has attracted more and more attention from researchers and production enterprises. Inkjet printing technology will be the inevitable development trend of digital printing [12,13,14,15,16,17,18].

Due to the function of fiber in the inkjet printing paper [19,20,21], the water resistance and antifouling performance of the inkjet printing surface are usually not ideal. In recent years, the hydrophobic/oleophobic surface of bionics has been extensively concerned by scholars [22,23,24,25,26,27,28,29,30]. Liquid resistance plays an important role in animals and plants as well as in real life. For example, the surface of the lotus leaf shows hydrophobic and self-cleaning effects. This hydrophobic performance is mainly due to its surface with low surface energy materials and a certain rough structure [22,23,24]. The wettability of a solid surface is the macroscopic characteristic of the interaction between liquid and solid matrix materials, and the contact angle (CA) is the most commonly used method to measure the wettability. From a geometric perspective, when the liquid forms an angle CA greater than 90° at the boundary of liquid gas solid three-phase intersection, it can be determined that the solid has hydrophobicity performance [25,26]. Hydrophobic/oleophobic material is a kind of bionic functional material with special wettability on the surface, which is based on the research of the "lotus leaf effect." Because it has the characteristics of self-cleaning, waterproof, antifouling, and corrosion resistance [27,28], it has broad development prospects in the fields of printing, packaging, electrical appliances, the chemical industry, national defense, and military affairs, etc. [29].

Post-press processing is an important and effective way to realize the unique function and value-added of printed matter. Aiming at the problem that inkjet printing is not hydrophobic and oleophobic, forming a hydrophobic and oil-repellent coating on its surface is an ideal method to solve this problem. Epoxy acrylic resin coating has many excellent properties, which include a wide range of raw materials and low cost, etc. Additionally, it is a kind of widely used coating. However, its water and oil resistance are not very good, and its film-forming adhesion and flexibility also need to be improved. By introducing fluorine-containing groups into the synthesis of resin polymers, fluorine-containing epoxy acrylic resin polymers can be obtained [30]. Due to the large electronegativity of fluorine and the very stable C-F bond [31], the modified polymer not only would maintain the original characteristics of epoxy acrylic resin polymer but also could improve the hydrophobicity and oil repellency of polymer coating effectively [32,33].

In this paper, based on E51 epoxy resin, fluorine-containing organic monomers were grafted onto the main chain of the resin molecule by grafting reaction. Then fluorine-containing epoxy resin was chemically modified by acrylic acid, maleic anhydride, and organic base in order, and finally, the fluorine-containing coating was prepared. The effects of the amount of fluorine monomer on the physicochemical properties of polymerization, film-forming surface hydrophobicity, and service properties were discussed, which would provide a useful reference for the design and synthesis of low surface energy polishing materials in the future.

2. Experimental Methods

2.1. Materials

The coating samples in this experiment were synthesized from dodecfluoroheptyl methacrylate (DFHMA), E51 epoxy resin, acrylic acid, maleic anhydride, triethylamine, ethanol, and purified water. The paper used for inkjet printing was 80 g/m² inkjet digital paper.

2.2. Equipment

The composition of reactive polymer molecules was determined by Fourier transform infrared spectrometer (FTIR), which was produced by Brooke company in Germany. The chemical composition of polymer film-forming surface was tested and analyzed by EscaLabXi+ X-ray photoelectron spectrometer, which was produced by Thermo Fisher technology. The hydrophobic and oleophobic properties of the copolymer film were tested by German Kruss dsa100s contact angle measuring instrument. In this experiment, TGAQ50 thermogravimetric analyzer of American TA instrument company was used to analyze the polymer film. The paint film adhesion and hardness of the polymer were tested by QFZ-Ⅱpaint film adhesion tester of Shandong Yuanwei instrument equipment Co., Ltd. (Jinan, China) and QHQ-A pencil hardness tester of China AIPLI instrument Co., Ltd. (Quzhou, China) respectively. The gloss of polymer film was tested by NHG26 gloss meter, which was produced by Shenzhen Sanenshi Technology Co., Ltd. (Shenzhen, China). The printing samples of this experiment were printed by Epson l15158 inkjet printer. Additionally, the color density value of the above samples was measured by the exact standard plate spectrodensitometer, of which brand is X-Rite.

2.3. Experimental Method

2.3.1. Sample Preparation

In this experiment, E51 epoxy resin was modified by grafting reaction, ring-opening reaction, esterification reaction, and neutralization reaction sequentially. Firstly, taking advantage of the high activity of the H atom on the methylene next to the ether bond on the main chain of epoxy resin, DFHMA was grafted onto the main chain of epoxy resin molecule under the action of initiator and the fluorine was introduced finally. Then, under the action of polymerization inhibitor and catalyst, acrylic acid reacted with epoxy group. In the next step, maleic anhydride, and the hydroxyl group, which was produced by the reaction of acrylic acid and epoxy resin, would be used to react and the carboxyl group would be introduced into the reaction system. To obtain fluorine-containing polishing coating, neutralization reaction for the acid and alkali would be carried out with neutralizer. Finally, the alkaline neutralizer was used for acid–base neutralization reaction, and the fluorine-containing coating would have been synthesized.

In this experiment, seven groups of epoxy group in epoxy resin and DFHMA with the molar ratio of 23:1, 24:1, 25:1, 26:1, 27:1, 28:1, and 29:1 were selected, which were recorded as samples ①–⑦, respectively. The inkjet printings, which were obtained by polishing and being coated with the corresponding coatings, were recorded as samples A-G, respectively.

2.3.2. Performance Tests and Characterization

Infrared Spectrum Analysis. The polymer synthesized in the experiment was determined and analyzed by Fourier transform infrared spectrometer of Brooke company in Germany. The instrument was turned on and preheated for about 30 min before the test, and then the reaction polymers were measured and analyzed. The numbers of scanning times were 30 and the measurement range was 500–4000 cm⁻¹.

Solid Content. Test the solid content of the sample according to ISO 3251:2003.

Grafting Rate. First, a certain amount of n-pentane would be added to the reaction product. After being shaken until it was fully dissolved, the upper of the n-pentane solution would be removed. The sample would be purified three times and would be placed at 100 °C in the oven until the weight would not change. The grafting rate would be calculated from the obtained data. The calculation formula is as follows:

$$P=\frac{G_1}{G_2}\times 100\%$$

(1)

In Formula (1), P is the grafting rate, and the unit is percentage (%). G₁ is the mass of the product after baking to constant weight in grams (g). G₂ is the mass of the product before purification in grams (g).

Membrane Thermogravimetric Analysis. The resin film was thermogravimetric analyzed by thermogravimetric analyzer in nitrogen environment, which was heated from 15 to 600 °C.

Chemical Composition Analysis of Membrane Surface. X-ray photoelectron spectroscopy (XPS) was used to test and analyze the surface chemical composition of the sample coated with coating.

Contact Angle. The hydrophobicity and oleophobicity of the coating film were tested and characterized by the contact angle measuring instrument.

Adhesion and hardness. The adhesion and hardness of the synthetic coating after film formation would be measured by relevant equipment according to the Chinese national standards GB/T 1720-1979 and GB/T 6739-2006, respectively.

Gloss. The gloss of the experimentally synthesized coating after film formation would be measured by gloss meter according to the international standard ISO 2813-2014.

Color Reproducibility. In order to evaluate the influence of coating on the image color reproduction performance of printing samples, the CIE L* a* b* values of inkjet printings before and after coating and glazing were measured by spectrophotometer. According to the measured values, the color gamut maps of all printed samples would be drawn, compared, and analyzed.

3. Results and Discussion

3.1. Infrared Spectrum Analysis

In this experiment, the structure of the reaction product was determined by the infrared spectrum, and the consistency between the actual reaction process and the theoretical design was analyzed. Firstly, the infrared spectrum of E51 epoxy resin without grafting reaction was measured by an infrared spectrometer. Then, after introducing fluorine monomer into the reaction system, the infrared spectrum of the polymer after the grafting reaction was tested. Finally, the difference in infrared spectra of reactants before and after the grafting reaction was compared to test whether fluorine monomer was introduced into the reaction system. The experimental test results are shown in Figure 1 and Figure 2.

Figure 1 is the infrared spectrum of E51 epoxy resin. It could be clearly seen from the figure that 1227.76 cm⁻¹ and 913.68 cm⁻¹ were the characteristic absorption peaks of epoxy groups in the epoxy resin. The absorption peak of hydroxyl in epoxy resin at 3438.55 cm⁻¹ was very weak. The characteristic absorption peaks of the benzene ring were 1607.10 cm⁻¹, 1580.58 cm⁻¹, 1507.16 cm⁻¹, and 1456.18 cm⁻¹. Figure 2 is the infrared spectrum of E51 epoxy resin after the introduction of fluorine. It could be seen from the figure that new absorption peaks appeared at 1182.89 cm⁻¹ and 1033.85 cm⁻¹, which were the peaks of -CF bonds, and 1295.06 cm⁻¹, which were the peaks of -CF₃ bonds. Due to the small amount of fluorine monomer, the intensity of the absorption peak was relatively weak, and 1731.51 cm⁻¹ was the stretching vibration peak of C=O bonds. In addition, 1237.95 cm⁻¹ and 913.68 cm⁻¹ were the characteristic absorption peaks of the epoxy group. Additionally, there was no stretching vibration peak of C=C bonds in the spectrum, which would indicate that the C=C bonds in dodecfluoroheptyl methacrylate had disappeared and successfully been grafted onto the resin.

The results of the infrared spectrum analysis showed that the structure of the reaction product and the process of reaction synthesis were consistent with the theoretical design. Next, the effect of the amount of fluorine monomer on the properties of the coating prepared in the experiment will be discussed.

3.2. Effect of Fluorine Monomer Content on the Properties of Coating Prepolymer

3.2.1. Grafting Rate

The number of reactants is one of the important factors that will affect the properties of reaction products. An appropriate amount of fluorine-containing monomer could make the reaction proceed smoothly, and the properties of reactants are stable. If the amount were too much, the reaction might be unstable, and even the grafting rate would not increase but decrease. Therefore, in this experiment, the molar ratios of the epoxy group in epoxy resin to DFHMA were 23:1, 24:1, 25:1, 26:1, 27:1, 28:1, and 29:1, respectively. The corresponding experimental samples were recorded as ①–⑦, respectively, and their grafting rates would be tested respectively. The experimental results are shown in Figure 3.

It could be seen from Figure 3 that the grafting rates of samples ④–⑦ were 77.9%, 76.5%, 70.3%, and 60.2%, respectively. With the increase in the amount of fluorine monomer, the overall trend of the grafting rate of the reaction system was to increase. Theoretically, increasing the amount of monomer in the reaction system could improve the grafting rate. The increase in the amount of fluorine monomer could increase the chance of grafting onto the main chain of epoxy resin molecules, which maybe can improve the grafting rate of the samples. However, when the molar ratios of the epoxy group in epoxy resin to DFHMA were less than 26:1, which was when the amount of fluorine monomer continued to increase, the grafting rate of the reaction system decreased, and the grafting rate of sample ① was only 30.2% especially. This might be because, with the increase in fluorine monomers, the self-polymerization of fluorine monomers occurred, which would lead to the failure of fluorine monomers to be grafted onto the main chain of epoxy resin molecules. Moreover, the increase in fluorine monomer content might reduce the efficiency of initiator free radical and macromolecular free radicals, which would be not conducive to graft polymerization. The above conditions should eventually lead to the reduction of the grafting rate of the samples.

From the data and results of this experiment, when the molar ratio of the epoxy group in epoxy resin to DFHMA was 26:1, the grafting rate of the sample was the highest.

3.2.2. Membrane Thermogravimetric Analysis

In order to determine the stability of the reactive polymer after the introduction of fluorine and the effect of the content of fluorine monomer on the stability of the reaction polymer, the polymers synthesized by different molar ratios of an epoxy group and DFHMA were tested by membrane thermogravimetric analysis. The test results are shown in Figure 4, in which the polymer sample without fluorine monomer was recorded as sample O.

It can be seen from Figure 4 that the synthesis (sample O) without fluorine monomer at that time had an obvious weight loss step at 330~420 °C, which indicated that the molecular chain in the synthesis had been decomposed at this stage. After the introduction of fluorine monomer, the thermogravimetric curve of the composite shifted to the right, which showed that the thermal stability of the composite was significantly improved. When the molar ratios of the epoxy group to DFHMA were greater than 26:1, which were samples ④–⑦, the thermal stability of the composite increased gradually with the increase in the amount of fluorine monomer. This might be because after the fluorine monomer molecular chain was introduced into the epoxy resin molecular main chain, the volume of the epoxy resin molecular main chain was larger than that before, which would make the synthetic molecules of that time, and later reaction becomes dense. At that time, the entanglement between molecules also became closer, which would improve the stability of the composite to a certain extent. When the introduction amount of fluorine monomer continued to increase, which were samples ①–④, the thermal stability of the composite decreased on the contrary. Although the introduction amount of fluorine monomer increased, as shown in Figure 3, its grafting rate decreased, which would make too many fluorine monomers stay in the polymer, and even the fluorine monomers may undergo self-polymerization reaction and stay in the products of the synthesis. These conditions should make the thermogravimetric curve of the synthesized prepolymer shift to the left with the increase in the amount of fluorine monomer.

From the data and results of this experiment, when the molar ratio of epoxy group in the epoxy resin to DFHMA was 26:1, the thermal stability of the sample was the highest, which was consistent with the analysis of the estimated value of the grafting rate in theory.

3.2.3. Effect of Fluorine Monomer Content on the Surface Hydrophobicity and Oleophobicity of Coating Film

In order to test the effect of fluorine monomer content on the hydrophobic and oleophobic properties of the synthetic coating polymer film, the contact angle of the coating polymer synthesized with different molar ratios of epoxy groups and DFHMA was tested. The test results are shown in Figure 5.

It can be seen from Figure 5 that with the increase in the introduction amount of fluorine monomer, the hydrophobic and oleophobic properties of the film of the coating prepolymer prepared in the experiment were significantly improved. When the molar ratio of the epoxy group to DFHMA was 29:1 (sample ⑦), the contact angle between the coating prepolymer film and water was 70.8°, and the contact angle between the coating prepolymer film and ethylene glycol was 35.3°. Moreover, when the molar ratio of the epoxy group to DFHMA was 26:1 (sample ④), the contact angle between the coating prepolymer film and water reached 121.5°, and the contact angle between the coating prepolymer film and ethylene glycol reached 80.9°. The hydrophobic and oleophobic properties of the coating prepolymer film were all greatly improved. However, when the introduction amount of fluorine monomer continued to increase (samples ①–③), the improvement of its contact angle would not be obvious. At this time, the fluorine content on the surface of the coating prepolymer might have reached saturation, and the contact angle was close to the maximum.

3.2.4. Effect of Adding Methods of Fluorine Monomer on the Surface Hydrophobicity and Oleophobicity of Coating Film

It was found that under certain other conditions, the addition mode of fluorine monomer also had a certain influence on the hydrophobic and oleophobic performance of the coating prepolymer film surface, which was prepared by the synthesis reaction in this experiment. In this experiment, continuous dropping and one-time two kinds of addition of fluorine-containing functional monomers were used to prepare coating prepolymers, of which their contact angle was tested. The test results are shown in

Table 1. Effect of adding methods of fluorine-containing monomers on contact angle of coating film.

Molar Ratios of Epoxy Group to DFHMA (Sample)	Adding Methods of DFHMA	Contact Angles (°)
		Water	Glycol
23:1 (①)	Continuous dropping	125	86
	One-time	103	71
24:1 (②)	Continuous dropping	124	85
	One-time	100	72
25:1 (③)	Continuous dropping	123	83
	One-time	110	71
26:1 (④)	Continuous dropping	122	81
	One-time	114	74
27:1 (⑤)	Continuous dropping	117	71
	One-time	110	65
28:1 (⑥)	Continuous dropping	108	56
	One-time	102	50
29:1 (⑦)	Continuous dropping	70	35
	One-time	67	30

.

It can be seen from Table 1 that when the method of adding fluorine monomer was continuous dropping, the contact angle of the prepared coating prepolymer film to water and ethylene glycol was greater than that of the coating prepolymer film, which was prepared by adding fluorine monomer at one time. Additionally, this phenomenon would be more obvious when the introduction amount of fluorine monomer was high such as in samples ①–③ especially. This is because when the fluorine monomer was introduced by continuous dropping, the fluorine-containing monomer could be evenly distributed in the polymer molecular chain. During the film-forming process of coating prepolymer, the fluorine-containing side chain could be evenly distributed on the film surface along with the main chain, which would improve the existence probability and stability of fluorine on the film surface to a certain extent. However, if the fluorine monomer were introduced by one-time addition, the fluorine-containing monomer would have instantaneous aggregation, which should lead to the self-polymerization of fluorine monomer and cannot be grafted on the main chain of the prepolymer. This might lead to the absence or uneven distribution of fluorine on the film surface during the film-forming process of coating prepolymer, which would lead to the decline and instability of hydrophobic and oleophobic performance of coating prepolymer film, finally. Combined with the analysis of the influence of fluorine monomer content on the reactive grafting rate, it might also be because too much fluorine monomer would make it more prone to self-polymerization. At the same time, if the dropping rate of monomer were too fast and the dropping time was too short, the concentration of fluorine monomer in the reaction system should be too large, which would reduce the grafting rate and even cause gel phenomenon. Therefore, in order to carry out the experiment smoothly and improve the grafting rate and the performance stability of the coating prepolymer, the continuous and slow dropping of fluorine monomer was selected in this experiment.

Combined with the above performance test and analysis experimental results, it can be seen that when the molar ratio of the epoxy group in epoxy resin to DFHMA was 26:1, which was sample ④, the performance of the prepared coating prepolymer was relatively stable and the hydrophobicity and oleophobicity of the original prepolymer film could be improved.

3.3. Chemical Composition Analysis of Membrane Surface

The C-F bond in organic fluorine compounds is short, and the bond energy is about 415~485 kJ/mol, which is higher than the destruction energy of ultraviolet rays. Moreover, due to the strong electronegativity of fluorine atoms, the modified polymer not only maintained the original characteristics but also improved the chemical inertness, weather resistance, pollution resistance, hydrophobicity, and oleophobicity of polymer coatings effectively. The most fundamental factor causing the different surface hydrophobic and oleophobic properties of fluoropolymer is the surface fluorine content, which is closely related to the introduction amount of fluorine monomer. Combined with the above test results, in this experiment, the sample ④ with the molar ratio of t of an epoxy group and DFHMA was 26:1 was selected as the test object, and the coating surface was analyzed by XPS. The results are shown in Figure 6.

It can be seen from Figure 6 that there were four elements of C, O, N, and F on the coating surface at that time, of which the relative content of fluorine accounted for 18.09%. The fluorine content on the coating surface was much higher than that before the introduction of fluorine monomer, which could greatly reduce the energy consumption of the coating surface. Through the tests of other samples, it was found that when the introduction amount of fluorine monomer was increased or reduced, the fluorine content on the surface of the coating would not increase. It showed that when the molar ratio of the epoxy group to DFHMA was 26:1, the fluorine content on the surface should reach saturation, and its night hydrophobicity may not increase. Such test results were consistent with its contact angle test results.

3.4. Effect of Fluorine Monomer on Performance of Coating

3.4.1. Hydrophobic and Oleophobic Performance

Through the above experiments, it can be seen that the fluorine-containing side chain could remain on the surface of the film during the drying film-forming process of the coating polymer, which could reduce the surface tension of the film and improve the hydrophobic and oleophobic performance of the coating significantly. In order to test the hydrophobicity and oleophobicity of the self-made fluorine-containing coating on inkjet printing, the self-made fluorine-containing coating (sample ④) and the coating without fluorine were coated on the inkjet printing paper, on whose surfaces the contact angles of water and ethylene glycol were tested. Additionally, the changes in the hydrophobicity and oleophobicity of the coating before and after fluorine modification would be compared. The experimental results are shown in Figure 7.

As can be seen from Figure 7, the surface tension of inkjet printing paper coated with coating with a fluorine element was reduced significantly, and the hydrophobicity and oleophobicity were improved greatly. In the test results, the contact angle of fluorine-containing coating on inkjet digital paper to water was 121.8° while the contact angle to ethylene glycol was 78.2°.

3.4.2. Adhesion and Hardness

The compatibility between coating and inkjet printing is also reflected in the mechanical properties of the coating on the surface of inkjet printing. In order to test the changes in mechanical properties of the fluorine-modified coating, this experiment took the coating prepolymer, which was with and without the introduction of fluorine, as the test object. Additionally, its adhesion and hardness would be tested and compared. The experimental results are shown in

Table 2. Adhesion and hardness of coating with and without fluorine modification (samples with different molar ratios of epoxy group to DFHMA).

	Mechanical Property	Adhesion (Grade)	Hardness (9B~9H)
Molar Ratio (Sample)
Coating Without Fluorine Modification	(O)	3	5H
Coating with fluorine modification	23:1 (①)	3	4H
	24:1 (②)	2	4H
	25:1 (③)	1	3H
	26:1 (④)	1	3H
	27:1 (⑤)	1	4H
	28:1 (⑥)	2	4H
	29:1 (⑦)	3	5H

.

According to the standard GB/T 1720, the adhesion grades are divided into seven grades, of which grade one is the best grade of adhesion, and so on, and grade seven is the worst grade of adhesion. Compared with the data in Table 2, it was obvious that the hardness of the coating prepolymer was reduced significantly, and the adhesion was improved significantly after the introduction of fluorine monomer. With the introduction of fluorine, the proportion of fluorine-containing flexible chains in prepolymer increased, which would improve the flexibility of the coating film and reduce its hardness. After the introduction of a fluorine-containing side chain, the coating prepolymer molecules might be more easily wetted and penetrated on the surface of inkjet digital paper, which can form a relatively firm coating. The fluorine-modified prepolymer contained polar groups, the fluorine element was mainly concentrated on the surface of the synthetic resin, and the epoxy main chain was still concentrated on the bottom layer of the film. This situation not only did not affect the adhesion of the film to the substrate, but the surface energy of the prepolymer would be reduced, and its flexibility and adhesion would be improved because of the introduction of the fluorine-containing flexible chain. However, DFHMA in the reaction system was prone to self-polymerization with the increase in fluorine content, which would lead to the instability of the prepolymer system and a decrease in adhesion.

3.4.3. Printing Gloss

Printing glossiness refers to the glossiness of paper after printing; that is, the degree of reflection at a certain angle after the surface of printed matter is irradiated by incident light. Good printing gloss is conducive to the reproduction and restoration of the color of printed matter and reduces the loss of color information. It is also an important parameter to evaluate the quality of printed matter. In this experiment, the coating prepolymer without fluorine element and the fluorine-containing coating prepolymer were coated on the inkjet printing, respectively, and its gloss was tested and compared. The experimental results are shown in Figure 8. Among them, the corresponding inkjet printings using fluorine-containing coating samples ①–⑦ were recorded as A–G, respectively. For example, inkjet printing A used coating ①, and printing G used coating ⑦. The inkjet printing without coating was recorded as sample P, and the printing using coating without fluorine was recorded as sample Q.

As can be seen from Figure 8, compared with the unpainted inkjet printing, whether the fluorine-containing coating or the coating without fluorine element were used, its gloss would be greatly improved. Among them, the gloss of sample D coated with the coating sample ④ was as high as 93, which was the most obvious improvement. Due to the existence of coating film on the surface of inkjet printing after coating, the surface would be flatter and smoother, and its gloss would also increase. Compared with the coating without fluorine, when the amounts of fluorine in the coating were small, which was the molar ratios of the epoxy group to dodecafluoroheptyl methacrylate were greater than 26:1, the gloss of inkjet printing would be improved but not obviously. The glossiness of printing samples A–C, which were coated with coating samples ①–③, respectively, was lower than that of printing sample D. This might be because, with the increase in fluorine monomer, it had self-polymerization and remained in the coating polymer, which would make the composition of coating prepolymer complex and unstable. Further, the film-forming performance, such as the flatness and smoothness of the coating polymer on the printed matter should be reduced, which then would reduce the glossiness of the printings. Moreover, the greater the amount of fluorine monomer in the coating used, the greater the reduction of the gloss of the corresponding inkjet printing, which also confirmed the above explanation.

3.4.4. Color Reproducibility

This experiment would calculate and compare the color gamut of inkjet printing samples before and after coating. The color gamut of a printed matter refers to the color range area that the printed matter can express. The larger the color gamut area of the printed matter, the better the color reproducibility of the printed matter. On the contrary, if the color gamut area were smaller, the narrower the range of color reproducibility would be. In this experiment, a square field of color blocks of the inkjet printing sample sheet pattern were designed, which included the following four colors: yellow (Y), magenta (M), cyan (C), and black (K). As shown in Figure 9.

Similarly, the corresponding inkjet printing samples using fluorine-containing coating samples ①–⑦ were recorded as A–G respectively. The inkjet printing sample without coating was recorded as sample P and the printing sample without fluorine-containing coating was recorded as sample Q. The L* a* b* values of the color block of each printing sample were measured by the color densitometer. According to the measured data, a* b* of C, M, and Y would be used as three points to draw the color gamut diagram of each printing sample. The calculation results are shown in Figure 10.

As can be seen from Figure 10, the color gamut of inkjet printing samples after coating was larger than that of inkjet printing samples without coating. Among them, the color gamut of inkjet printing sample D, which was coated with coating sample ④, was the largest, and the improvement of its color reproducibility was the most obvious. The use of polishing oil made the surface of inkjet printing flatter and smoother. The color information of ink could be obtained through reflection to a greater extent, which would increase the measured value of color density. Additionally, the color gamut of the corresponding inkjet printing sample would also increase simultaneously. The introduction of fluorine made the coating prepolymer film flatter and smoother, especially when the amounts of fluorine monomer were small. If the amounts of fluorine monomer were too large, the coating film would be not flat and smooth, which might make the color part of the ink disperse through refraction and projection. The measured value of its color density should be reduced and the color gamut of the corresponding printing would become narrower, which would affect and weaken the image color reproduction ability of the picture and text.

Through the above tests and the analysis and comparison of the test results, it is recommended to use the coating with the molar ratio of an epoxy group and dodecyl fluoroheptyl methacrylate being 26:1 for inkjet printing, which could improve the properties of printing in hydrophobicity, oleophobic, surface mechanics and color reduction. Additionally, the quality of the inkjet printing would be simultaneously improved.

4. Conclusions

The experimental results show that when the molar ratio of the epoxy group in epoxy resin to DFHMA was 26:1, the coating prepolymer (sample ④) system, which was synthesized by reaction, had better comprehensive performance, higher fluorine grafting rate, and higher thermal stability. At the same time, the mass fraction of fluorine on the film surface of the sample reached 18.09%, which greatly improved the possibility of its hydrophobicity and oleophobicity. In terms of practical performance, the self-made coating prepolymer (sample ④) had good adhesion and flexibility on inkjet printing and had good coating adaptability with the inkjet printing. The hydrophobicity and oleophobicity of the printed matter after coating had been improved greatly. Especially in the aspect of hydrophobicity, its contact angle with water was 121.8°, which was very close to the realization of a superhydrophobic phenomenon. At the same time, in terms of printing quality evaluation parameters such as performances of glossiness and color reproduction, the inkjet printings had been improved too.

Experiments showed that the self-made coating could be used in inkjet printing, and its effect on the functional modification of the surface of inkjet printing had been realized. Different from previous studies, the performance testing and characterization methods used in this study were aimed at the performance of fluorine-modified coatings. Previous studies had focused more on the mechanism of fluorine modification and the performance of modified coatings, but there were few methods for testing and characterizing the actual application performance of the fluorine-modified coatings. The test methods and analysis results in this paper would directly feedback on the good or bad application effect of fluorine-modified coatings and would provide an important reference value for the later pilot production and application of modified coatings. However, it can be seen from the experiment that the oleophobic performance of the coating needs to be improved, while its physical and chemical properties and service performance need to be further studied. Additionally, its mature and stable synthesis process maybe also need to be determined through more refinement experiments.