Paper is one of the most widely used materials, due to its application in books, packaging, and office supplies. However, conventional paper has many disadvantages, such as low tensile strength, poor resistance to water and oil contamination, lack of antigraffiti capability, and easily deformed by moisture. Therefore, we proposed a double-layer protective coating that consists of a polyurethane (PU) bottom layer, to enhance tensile strength, and a top polydimethylsiloxane (PDMS) composite layer to enable antigraffiti and contamination-resistant capabilities. In the study, a double-layer coating was applied to conventional paper, using a simple two-step spraying method. The results showed that the prepared coating structure exhibited outstanding self-cleaning characteristics with respect to both water and oil contamination and adequately demonstrated antigraffiti capability. Moreover, the coating maintained superhydrophobic and oil-stain resistance after four months of outdoor storage. Finally, the tensile strength of the coated paper was 6.3 times as high as that of the original paper, making this coating structure a promising solution to the problems that limit paper from being more widely utilized in commercial and consumer application.
Paper is a widely used material throughout numerous commercial industries and by individual consumers, owing to its versatility, low cost, extensive production, and short processing time. However, despite the high demand, paper use is somewhat limited due to the following factors: (1) paper is composed of countless fibers that are inconsistently applied. Due to variations in fiber size and arrangement, copious gaps develop between the fibers. As such, when paper comes in contact with water, the water molecules are absorbed along the gaps, making it vulnerable to moisture and difficult to preserve over long periods of time [
Because the results of these earlier attempts to develop superhydrophobic coatings do not fully protect the paper, alternative solutions continue to be investigated. Dimitrakellis et al. [
Another method of preventing paper from being damaged by water- and/or oil-based substances consists of preparing superamphiphobic paper via plasma processing [
In an attempt to mitigate the shortcomings of conventional paper, we propose a double-layer protective coating composed of a PU bottom layer and a PDMS top composite layer. When the coating was applied to the paper with a simple two-step spraying process, it was shown to be self-cleaning and transparent and has high antigraffiti performance and enhanced strength.
Deli Co. provided 7465 copy paper. The 98% AR toluene, 5
A 524 G magnetic stirrer, produced by the Shanghai Mei Ying Pu Instrument Manufacturing Co., and a 500 W ultrasonic vibrator, manufactured by Hangzhou Frante Ultrasound Technology Co., were used to disperse the solution. A W71, ANEST IWATA Corporation, spray gun was employed for solution spraying. A Japan Hitachi Group S3000 scanning electron microscope (SEM) and a BX53M Olympus Corporation light microscope were used for surface topography characterization. An atomic force microscope was used to detect physical morphology (Bruker Dimension Icon, Brook) with HQ-300-Au (Ti/Au Coated Tips, 40 N/m, 300 kHz). The water contact angles (CAs) on all the surfaces were measured using the JC2000C1 contact angle measurements system—manufactured by Shanghai Zhongchen Digital Equipment. Olympus Corporation LEXT OLS4000 3D Measuring Laser Microscope is used to measure the thickness of coatings. A UTM2203, 0-5000N series Single Column Computerized Electronic Universal Testing Machine, produced by Zhenbang Testing Machinery Factory, was used for tensile strength analysis.
Ten milliliters of PU was added to the spraying device and sprayed on the paper’s surface for 6 sec at a distance of 30 cm. The coating was then left to cure at room temperature for 4 hr, or heated at 80°C for 20 min, to reach a semicured state.
Zero point two grams of SiO2 was mixed with 10 ml of toluene reagent and sonicated for 10 min. Next, 0.05 g of PTFE particles was added and the solution was sonicated for an additional 20 min to form a uniform dispersion. One gram PDMS and 0.1 g curing agent were mixed into the solution, which was magnetically stirred for 10 min. The mixture was then set aside and designated Solution A. Next, Solution B was composed by mixing 1 ml perfluorodecyltrimethoxysilane with 10 ml alcohol and magnetically stirred for 20 min.
Solution A was sprayed on the semicuring PU strength coating for 6 sec at a distance of 30 cm. The coating layer was left to cure at room temperature for 4 hr, or heated at 80°C for 20 min, to reach a semicured state. Next, Solution B was sprayed on the semicuring Solution A layer for 3 sec at a distance of 30 cm. Curing was then resumed either at room temperature for 20 hr, or by heating at 80°C for 1.5 hr, to fully cure the coatings on the paper.
Figures
Images of paper pattern before and after spraying: (a) photograph before spraying, (b) photograph after spraying, (c) optical microscope and SEM before spraying, (d) optical microscope and SEM after spraying, (e) AFM image of coating before spraying, (f) AFM image of coating after spraying, (g) height profile of coating before spraying, and (h) height profile of coating after spraying.
Before spraying
After spraying
Before spraying
After spraying
Before spraying
After spraying
Before spraying
After spraying
Figure
In addition, EDS analysis was carried out to detect any changes in element composition and/or concentration between the “before” and “after” samples. The experimental results are shown in Figure
EDS elemental analysis of the paper sample before and after coating application.
Before application of the coating
After application of the coating
Figure
A water drop test, employing blue pigmented water, was used to assess the wettability of the original sample and a paper sample coated with Solution A (Figure
Water droplets on the original paper and coated paper samples.
The water drop on the original sample was hemispherical in shape and over time absorbed into the paper, causing the paper to become damp and deformed. Contrarily, the water drop on the paper with Solution A coating was spherical in shape, and no water was absorbed over time. These results were quantified using CA measurements, which are shown in Figure
Contact angle images of water drops on the original sample and the sample with Solution A coating.
Original paper sample over time
Paper sample with coating of Solution A
Figure
To gain further insight into the effects of PDMS, PTFE, and SiO2 on coating wettability, the three Solution A components were individually investigated and the results are shown in Table
Effects of Solution A components—PDMS, PTFE, and SiO2—on the coating wettability (in 10 ml toluene).
Coating | PDMS (g) | PTFE (g) | SiO2 (g) | Contact angle (°) |
---|---|---|---|---|
Original paper | 0 | 0 | 0 | 85.0 |
A1 | 1 | 0 | 0 | 117.5 |
A2 | 1 | 0.05 | 0 | 130.8 |
A3 | 1 | 0 | 0.2 | 140.3 |
A | 1 | 0.05 | 0.2 | 152.1 |
Based on the results in Table
Graffiti damage is a major problem in cities throughout the world, so there is value of investigating and understanding the antigraffiti properties of paper. Thus, the antismudge capability of our paper coatings was investigated and the results are shown in Figure
Effects of antigraffiti on paper with two coatings.
Figure
The antigraffiti capability was further characterized by examining coatings of varied compositions and properties. The results from this investigation are depicted in Table
Antigraffiti effect of paper with different coatings.
Coating | Solution B spraying | Superhydrophobic | Antigraffiti effect |
---|---|---|---|
A1 | No | No | No |
A2 | No | No | No |
A3 | No | No | No |
A | No | Yes | No |
A1B | Yes | No | No |
A2B | Yes | No | No |
A3B | Yes | No | No |
AB | Yes | Yes | Yes |
As shown in Table
In this experiment, a mixture of oil and carbon black was used to simulate oil contamination. The mixture was dropped onto the surface of the original paper and the paper coated with a PDMS composite coating. The process was videotaped from start to finish and Figure
Photos of oil contamination dropped on the original paper: (a)
As shown in Figures
The application of paper is limited because its low tensile strength allows it to be easily torn or damaged. In order to improve the strength of conventional paper, a PU strength coating layer was added between the PDMS composite layer on top and the paper on the bottom. Figure
Complete structure of the double-layer coating.
Moreover, LEXT 3D Measuring Laser Microscope was used to measure the thickness of the whole coating (PDMS composite and PU strength layer). Make the coating into a step pattern and measure the height of the step, and one of the measurements is shown in Figure
Measurement of coating thickness.
The results in Figure
In order to test the tensile strength of the paper, both with and without coatings, the paper samples were cut into
Relationship between tensile strength and time of the three kinds of paper.
Figure
The transmittance of the entire coated structure, including the PDMS composite coating and the PU strengthener coating, was inspected on glass using an ultraviolet-visible spectrophotometer. Results are shown in Figure
Transmittance on bare glass and on glass with whole coatings.
The maximum transmittance of the bare glass was 91.19% in the visible wavelength range of 350-800 nm, while the maximum transmittance of the glass with the entire coating structure was ~70% in the visible wavelength range. These results indicate that the coatings have adequate transmittance for message expression on paper.
The coatings weighing 200 g were placed facedown on a sandpaper with 800 meshes and pushed back and forth for 10 cm, which is a cycle. After each cycle, the contact angle is measured and the testing results are shown in Figure
The wear-resistant test process device; water/oil CA as a function of the number of abrasion cycles for coatings.
The testing results show that after 10 abrasion cycles, the coating still keeps superhydrophobic and resistance to oil contamination, indicating that the coating has the outstanding wear-resistant property.
To determine the durability of these manufactured paper coatings, the paper sample was covered with the entire coating structure and placed in an outdoor environment. The durability of the coating was determined by measuring the static water and hexadecane CAs once a month for four months. CA results are depicted in Figure
Changes in water and hexadecane on the coated paper over time.
Figure
In this study, a transparent multifunctional double layer was prepared and applied to paper using a simple two-step spraying process that implemented PU as a strength layer and composite PDMS as an anticontamination layer. The coating structure changed the paper’s properties, so that it was no longer easily deformed by moisture, contaminated by graffiti and/or oil, or effortlessly torn. Compared with similar studies, PU and PDMS are used as the main materials as the strength layer and the antigraffiti functional layer, respectively. These two materials have low cost, nontoxicity, simple process, and good light transmittance. The coated paper has good self-cleaning properties with respect to both water and oil contamination and adequate antigraffiti capability. Moreover, the coating still maintained self-cleaning properties after four months of outdoor storage. Finally, the tensile strength of the coated paper is 6.3 times as high as that of the original paper sample; and thus, the coating structure should be potentially considered for expanding paper’s application in commercial industry.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was funded by the National Natural Science Foundation of China (Grant No. 51475353), the Natural Science Basic Research Program of Shaanxi Province (Grant No. 2016JM5004), and the Key Laboratory of the Shaanxi Provincial Department of Education (Grant No. 16JS057).