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FULL PAPER Multifunctional Synthetic Paper

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Polyurethane Synthetic Papers Based on Different Inorganic Fillers with Water and Fire Resistance Hu Zhou, Xiaohong Wang, Taofen Wang,* Jian Jian,* Zhihua Zhou, Jianxian Zeng, Lingwei Zeng, and Guoqing Liu serious water pollution.[3,4] According to surveys, water consumption for paper­ Polyurethane synthetic papers based on different inorganic fillers are premaking ranged from 5 to 300 m3 per ton pared via wet phase inversion process and inorganic mineral filling modificaof pulp products in one factory, leading tion technique. The surface and cross-section morphology, tensile strength to about 2000 m3 of effluents per day in and elongation at break, fire resistance, water and oil adsorption, ink contact recent years.[5] Meanwhile, preparation angle, writing and printing effects, and water resistance of synthetic paper of traditional paper consumes a large number of timbers, resulting in serious are investigated, respectively. A large number of pores are present on the soil erosion. What is more, it cannot be surface and in the interior of the synthetic papers and provide huge space for ignored that traditional papers have hardly ink molecules. The synthetic paper with transparent powder filled exhibits the satisfied the public growing demands optimal performance in tensile strength and elongation at break. Moreover, along with rapid industrial development. the fire resistance of polyurethane is improved by inorganic particles, and Therefore, a novel, greener, and multi­ functional paper is urgently needed. the highest limit oxygen index value of synthetic paper reaches up to 31.8%, Recently, the synthetic papers made which meets the standard of flame retardant. Meanwhile, filled synthetic of polymer resin and inorganic particles papers exhibit high water and oil adsorption capacity, good ink-affinity, have attracted significant attention because excellent writing and printing effects. In addition, filled synthetic papers they can greatly reduce wood consump­ perform well in water resistance when written and printed synthetic papers tion and weaken negative impacts on the are immersed in water. This work may provide a new idea to design a novel, environment.[6] In previous work, many researchers have focused on using of poly­ greener, and multifunctional synthetic paper for its broad applications. propylene, polystyrene, polyester, poly­ ethylene, and polyvinyl chloride as polymer resins to prepare synthetic paper.[7] However, some problems 1. Introduction arise when these polymer resins are used for fabricating syn­ thetic paper, such as low polarity, low surface tension, poor wet­ Traditional papers which are made of woods have been widely tability, poor adhesion for ink, poor biodegradability, and white used in daily life, such as newspapers, books, packing paper, pollution, which seriously hinder their practical applications.[8] office paper, and advertising paper. However, owing to the advantages of low price and convenience, a huge number of Thermoplastic polyurethane (TPU), as a new type of polymer traditional papers are consumed and wasted.[1,2] And most of resin, is composed of alternately linked soft and hard segments and is widely used in the leather, medical, cosmetic, food, and the waste papers are thrown away optionally and treated as gen­ coating industries owing to their superior mechanical proper­ eral garbage, leading to the waste of non-renewable resources ties, durability, abrasive resistance, flexibility, elasticity, etc.[9–12] and serious environmental problems. In addition, chemical additives and bleaching agents used in papermaking also cause Moreover, TPU also exhibits unique biocompatibility and biodegradability due to the abundant hydrophilic functional groups, such as amino (NH2) and carboxyl (COOH).[13] Therefore, TPU can act as an environmentally friendly raw Prof. H. Zhou, X. Wang, T. Wang, Dr. J. Jian, Prof. Z. Zhou, Prof. J. Zeng, Dr. L. Zeng, Dr. G. Liu material in synthetic paper fabrication; whereas, the poor flame Key Laboratory of Theoretical Organic Chemistry and Function Molecule resistance of TPU extremely limits their practical applications Ministry of Education in a broad variety of fields.[14] In recent years, the inorganic Hunan Provincial Key Laboratory of Controllable Preparation mineral filling modification technique has attracted significant and Functional Application of Fine Polymers School of Chemistry and Chemical Engineering attention. By filling inorganic minerals in polymers, the prop­ Hunan University of Science and Technology erties of polymers can be effectively improved, and the cost of Xiangtan 411201, China raw materials can be greatly reduced at the same time. Con­ E-mail: [email protected]; [email protected] cretely, montmorillonite,[15–17] calcium carbonate,[18,19] titanium The ORCID identification number(s) for the author(s) of this article dioxide[20,21] and silica[22,23] are frequently used as inorganic can be found under https://doi.org/10.1002/mame.201800473. minerals to improve the properties of polymers in flame resist­ ance, rigidity, opacity, dimensional stability and ink receptivity. DOI: 10.1002/mame.201800473

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Hence, inorganic mineral filling modification is a feasible method to combine the advantages of TPU and inorganic min­ erals to prepare synthetic paper. Traditional methods for synthetic paper making mainly include calender method, blown film method, casting method, and biaxial stretching method, which are complex, timeconsuming, and need specific equipment and multistep pro­ cedures.[24,25] It is well known that the wet phase inversion is one of the most efficient and convenient methods to prepare polymer films under normal temperature and pressure condi­ tions with simple equipment. In this research, a ternary mixed system, containing TPU, dimethylformamide (DMF), and inorganic particles (diatomite powder, light calcium carbonate powder, egg­ shell powder, zeolite 13X powder, and transparent powder) is applied to prepare synthetic paper by combining the wet phase inversion process and the inorganic mineral filling modification technique. Different inorganic particles are added as modifying agents to explore the effects on the prop­ erties of synthetic papers caused by different inorganic parti­ cles. The main purpose of our work is to investigate how the different inorganic particles influence the morphology, tensile property, fire resistance, water and oil adsorption capacity, ink contact angle, writing and printing performance, and water resistance of synthetic papers, and simultaneously to provide a new idea to prepare a novel, greener, and multifunctional synthetic paper.

2. Experimental Section 2.1. Materials TPU (molecular weight of 105, shore hardness of 90 A) was provided by the National Engineering Laboratory for Clean Technology of Leather Manufacture in Sichuan University (Chengdu, China). DMF (analytical reagent [AR]) was obtained from Kelong, Co., Ltd. (Guangdong, China). Light calcium

carbonate powder (CaCO3) was purchased from Fujian Yonghang Trading Co., Ltd. (Fujian, China). Transparent powder was kindly supplied by Shenzhen Haiyang Powder Technology Co., Ltd. (Shenzhen, China). Zeolite 13X powder was pro­ vided by Jiangxi Xintao Technology Co., Ltd. (Jiangxi, China). Diatomite powder was purchased from Shanghai chemical reagent procurement station reagent factory. Eggshell powder was made in laboratory. Oleic acid (C18H34O2, AR) was pur­ chased from Xilong technology Co., Ltd. (Guangdong, China). The size distribution of inorganic fillers was measured using a Malvern Mastersizer 2000 laser diffraction device (Malvern Instruments Ltd., Malvern, UK). By taking wet sieving process with three replicates, the d(0.5) of diatomite powder, light cal­ cium carbonate powder, eggshell powder, zeolite 13X powder, and transparent powder were 31.514, 3.176, 9.375, 4.685, and 12.166 µm, respectively. The value of d(0.5) represents the par­ ticle size corresponding to the cumulative particle size distri­ bution percentage of the sample reaching 50%.

2.2. Preparation of Polyurethane Synthetic Papers Based on Different Inorganic Fillers Polyurethane synthetic papers based on different inorganic fillers were prepared via wet phase inversion process and inorganic mineral filling modification technique. The process of synthetic paper preparation is presented in Figure  1. Typi­ cally, TPU was completely dissolved in DMF solution with the proportion of 3:11 (wt%) by magnetic stirrer at 30–40 °C for 6 h. Different inorganic fillers (diatomite powder, light cal­ cium carbonate powder, eggshell powder, zeolite 13X powder, and transparent powder) were added to the polymer solution and the mixture was stirred using electric mixer until a ternary homogeneous mixture was obtained. After leaving the tertiary mixture for 2 h to let the bubbles disappear, the tertiary mixture was cast onto a cellophane paper and then pushed using a bar to get a flat coating film. Subsequently, the cellophane paper with coating film was immersed quickly into deionized water

Figure 1.  Synthetic paper preparation process.

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2.3.3. Fire Resistance Test Flammability of the synthetic papers was investigated by Limit Oxygen Index and Vertical Burning Test (JF-3, Nanjing, China) according to GB/T 2406.2-2009 standard.

2.3.4. Water and Oil Adsorption Figure 2.  Frame structure of synthetic paper.

at room temperature for 1 h. During the wet phase inversion process, the DMF was fully replaced by deionized water and composite materials were precipitated to form synthetic paper. The synthetic paper was washed with deionized water three times to remove excessive solvents and then dried in the drying chamber at 40 °C for about 5 h. In order to study the effects of inorganic fillers on synthetic paper properties, standard syn­ thetic paper without filler was made. A schematic of the basic frame structures of synthetic papers is shown in Figure 2. TPU served as a basic framework of synthetic paper and different inorganic particles served as modifying agents to improve the properties of synthetic paper. The synthetic papers were cut to A4 size and denoted as TPU-N, TPU-S, TPU-C, TPU-E, TPU-Z, and TPU-T, respectively. Detailed information is listed in Table 1.

Deionized water and oleic acid were adopted to test water and oil adsorption. The synthetic papers were cut to 30 mm length and 30 mm width, respectively, and then placed in a vacuum oven at 40 °C until constant weight. The dried samples were soaked in deionized water and oleic acid solutions for 24 h at room temperature. Filter papers were used to wipe the solution on the surface of samples after soaking. The papers were then collected and weighed. Water and oil adsorption ratios (γwater, γoil) were used to deter­ mine water and oil adsorption. The values of γwater and γoil after soaking were calculated by Equations (1) and (2), where m0 is the mass of synthetic paper before soaking and m is the mass of synthetic paper after soaking.

γ water = (m − m 0 )/m 0 × 100 γ oil = (m − m 0 )/m 0 × 100

 

(1) (2)

2.3. Synthetic Paper Characterization 2.3.1. Morphology

2.3.5. Ink Contact Angle Test

The surface and cross-section morphology of the synthetic papers were observed using scanning electron micros­ copy (SEM, Hitachi Model S-520, Japan). The samples were fractured in liquid nitrogen, and then coated with a thin layer of gold palladium alloy to produce electrical conductivity.

The ink contact angles were measured on an optical contact angle system (JC2000D1, Shanghai, China) at room tempera­ ture using a 3 µL deionized water droplet on at least five dif­ ferent locations on each paper.

2.3.6. Writing and Printing Test 2.3.2. Tensile Test The tensile property of synthetic papers was conducted at room temperature with an electronic tensile machine (GT-A1-7000S, Taiwan, China) under a tensile speed of 20.0 mm min−1. The samples were in the shape of dumbbell and each sample was tested three times.

Writing and printing performances of TPU/inorganic particle synthetic papers were evaluated by the 0.5 mm black ink pen of Chenguang (Chenguang Co., Ltd. China) and Canon IP2000 ink-jet printer (Canon Co., Ltd. Japan), respectively.

2.3.7. Water Resistance Test Table 1.  Composition of synthetic papers. Sample

Filler type

Filler [g]

TPU [g]

DMF [g]

Filler solids [wt%]

TPU-N

None

0

15

55

0

TPU-S

Diatomite powder

22.5

15

55

60

TPU-C

Light CaCO3 powder

22.5

15

55

60

TPU-E

Eggshell powder

22.5

15

55

60

TPU-Z

Zeolite 13X powder

22.5

15

55

60

TPU-T

Transparent powder

22.5

15

55

60

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The synthetic papers used in water resistance test were written and printed in advance, and then dipped completely in deion­ ized water for 1 week at room temperature.

3. Results and Discussion 3.1. Morphology of Synthetic Paper The surface and cross-section morphology and microstructure of synthetic papers are presented in Figure  3. Obviously, SEM images of synthetic papers with fillers showed dramatically

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changed morphology compared to reference synthetic paper with no filler added. As shown in the first row of Figure 3, the surface of syn­ thetic paper without filler is more homogeneous and featureless than those of the synthetic papers with fillers. Diatomite, light calcium car­ bonate, eggshell, zeolite 13X, and transparent powder were clearly observed on the surface of synthetic papers. However, among these synthetic papers, inorganic particles on the surface of TPU-S, TPU-C, TPU-E, and TPU-Z exhibit a certain degree of agglomeration. The surface morphologies might be due to the instantaneous demixing of casting solution and rapid precipitation of polymer matrix. The high affinity between solvent (DMF) and non-solvent (deionized water) is facilitated to reach the phase demixing line when the coating films are immersed in coagulation bath.[26] The crosssectional SEM images of prepared synthetic papers are presented in the second and third rows of Figure 3. It can be observed that the synthetic papers had an asymmetric structure consisting of a top layer, a finger-like struc­ ture, and a spongy structure. From the high magnification cross-sectional images, different inorganic particles were clearly observed inside the synthetic papers and a portion of finger-like structures extends from the top to the bottom. However, with inorganic fillers added, synthetic papers shown more numerous shorter fingerlike structures than the synthetic papers without fillers. In the forming process of cross-section morphologies, the presence of inorganic fillers in the casting solution greatly hindered the exchange of DMF and deionized water, which delayed liquid–liquid demixing.[26,27] Owing to this hindrance effect, inorganic particles can act as a growing point of new finger-like structures to form more finger-like structures in the syn­ thetic paper forming process.

3.2. Tensile Test of Synthetic Paper

Figure 3. The surface and cross-sectional morphologies of synthetic papers: a) TPU-N, b) TPU-S, c) TPU-C, d) TPU-E, e) TPU-Z, and f) TPU-T.

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The mechanical properties of pure and filled synthetic papers are given in Figure 4. It can be revealed that the tensile strength of synthetic papers tended to decrease as different inor­ ganic particles with a solid content of 60 wt% filled. The tensile strength of TPU-S, TPU-C, TPU-E, TPU-Z, and TPU-T decreased from 2.36 to 1.29, 1.98, 2.18, 1.59, and 2.29 MPa, respectively. And the elongation at break of synthetic papers with fillers also performed poor except for the sample of TPU-T. The elongation at break of TPU-S, TPU-C, TPU-E,

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www.mme-journal.de Table 2.  Limit oxygen index (LOI) value of synthetic papers. Synthetic paper

Figure 4.  Tensile properties of synthetic paper.

and TPU-Z decreased from 157.25% to 134.57%, 65.86%, 51.54%, 135.61%, and 192.46%, respectively. As we all know, the dispersion of inorganic particles in organic materials has a great influence on the properties of organic–inorganic com­ posite materials.[28] The results may be ascribed to the agglom­ eration of inorganic particles with large size scale formed in the prepared papers. Thus, the specific surface area of inorganic particles greatly decreased, resulting in the decrease of inter­ facial interaction between fillers and polymer.[29] The agglom­ erate phenomena were observed from SEM images of prepared samples. In addition, due to the non-uniform distribution of diatomite, light calcium carbonate, eggshell, and zeolite 13X powder, uneven stress in the filled synthetic papers appeared when an external force was applied in experiment, which also led to a decrease of the tensile strength and elongation at break.[30] Among these filled synthetic papers, TPU-T showed the highest tensile strength and elongation at break. Com­ pared with TPU-N, the elongation at break of TPU-T exhibited an improvement by 35.21%. The reinforcement effect may be attributed to uniform distribution of transparent powder in the synthetic paper. And because of uniform distribution, the transparent powder had a higher surface area, which enhance the connection between transparent powder and the chains of TPU with physical and chemical forces.[31] According to the literature, compared with single polymer material, the tensile strength and elongation at break of the polymer matrix have certain improvement when a suitable amount of inorganic particles filled into polymer.[32,33] However, in this preparation system, the synthetic paper with a solid content of 60 wt% cause the decrease of mechanical properties. It can be indi­ cated that a more suitable solid content should be studied in the future works to improve the mechanical properties of filled synthetic papers.

3.3. Fire Resistance of Synthetic Paper The limit oxygen index (LOI) is a standard test method to measure the minimum oxygen concentration. Specifically, the material with LOI value 27% reveals flame retardant material.[34] The LOI values of prepared synthetic papers are presented in Table  2. As one of the main raw materials of synthetic papers, TPU exhibited poor fire resistance, which greatly hindered its practical applica­ tions. As shown in Table 2, the LOI value of TPU-N was 19.7% which demonstrated that it was a flammable material. Filling fire retardancy materials in polymer matrix, such as inorganic particles, is the most commonly used method in fire retardant modification. Compared with TPU-N, the LOI values of TPU-S, TPU-C, and TPU-E increased from 19.7% to 21.3%, 21.7%, and 21.7%, showing an improvement by 1.6%, 2.0%, and 2.0%, respectively. These results suggested that the fire resist­ ance of synthetic papers had a little improvement when diato­ mite, light calcium carbonate, and eggshell powder were used alone in paper making. In addition, the LOI value of TPU-T was increased from 19.7% to 24.4%, which changing TPU-T from flammable material to combustible material. It was worth noting that the LOI value of TPU-Z was significantly increased from 19.75% to 31.8%, exhibiting an improvement by 12.1% when zeolite 13X powder was added into synthetic paper. Obvi­ ously, TPU-Z reached the requirements (LOI > 27%) of refrac­ tory material. It was indicated that zeolite 13X powder can efficiently improve the flame retardancy of TPU. In summary, the fire resistance of prepared synthetic papers all had a certain degree of improvement after filling different types of fillers. The improvement effects may be ascribed to the inorganic particles on the surface and in the internal pores of synthetic papers obstructed air flow and slowed down the speed of heat diffusion. Zeolite 13X, consisting of SiO4 and Al2O3, is a kind of crystalline aluminosilicate with uniform pore structure, large specific surface area, and high surface polarity, which also per­ formed well in thermal stability and adsorption capacity.[35] The uniform pore structure and large specific surface area of zeolite 13X played a role in forming an effective expanded carbon layer at high temperature, which also is one of the important factors to enhance the flame retardancy of synthetic paper.[36] Moreover, it can be clearly observed from the cross-section morphology at high magnification that TPU-Z possesses lesser spongy struc­ tures than other prepared synthetic papers. The less spongy structures can greatly decrease oxygen flow, and then enhance the fire resistance of TPU-Z. The asymmetric porous structure of synthetic paper and high adsorption capacity of zeolite 13X were facilitated to absorb poisonous smoke and gases to pro­ tect people from health damage when a fire broke out in daily life. Owing to the best fire resistance compared with all filled synthetic papers, TPU-Z was more suitable to prevent flam­ mable objects from fire destruction, such as vital information

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Figure 7.  Writing performance of synthetic paper: a) TPU-N, b) TPU-S, c) TPU-C, d) TPU-E, e) TPU-Z, and f) TPU-T. Figure 5.  Water and oil adsorption of synthetic paper.

documents, wallpaper, important records, calligraphy, and paintings of celebrities.

3.4. Water and Oil Adsorption Water and oil adsorption were measured to investigate the influence of inorganic fillers on synthetic papers. The results are summarized in Figure 5. As can be seen, the water and oil adsorption ratio of TPU-N was higher than other filled syn­ thetic papers except for TPU-S. It can be concluded that a good deal of porous structures of synthetic paper provided channels for water and oil adsorption. However, the adsorption capaci­ ties of TPU-C, TPU-E, TPU-Z, and TPU-T were not satisfactory. Although inorganic particles can increase the capacity of water and oil adsorption, inorganic particles occupied the majority of spaces, resulting in the great decrease of spaces for water and oil adsorption. TPU-S exhibited better water and oil adsorp­ tion performances compared with other filled synthetic papers. The main component of diatomite powder is SiO2 and it has unique diatom shell structure and high porosity, which contrib­ utes to excellent adsorption performance.[37] Hence, TPU-S may

Figure 6.  Ink contact angle of synthetic paper.

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play an important role in sewage treatment and oil-absorbing sheets. It was worth noting that zeolite 13X as well had uni­ form pore structure, nevertheless, its water and oil adsorption capacity was inferior to TPU-S. This result may be attributed to the lesser amount of spongy structures of TPU-Z than TPU-S, and the specific structure of papers can be observed from SEM images.

3.5. Ink Contact Angle of Synthetic Paper Generally, improved ink-affinity means better writing and printing performances for the synthetic paper. In this research, the ink-affinity of all synthetic papers surfaces was character­ ized by contact angle goniometer. Lower ink contact angle meant better synthetic paper surface ink-affinity. The ink con­ tact angle data are presented in Figure  6. All the ink contact angles were less than 90°, which means synthetic papers pos­ sessed good ink-affinity. The strong hydrophilic nature of abundant COOH groups in the polyurethane may have con­ tributed to the good ink-affinity of synthetic papers. It could be clearly observed that the synthetic paper without filler pos­ sessed the highest ink contact angle of 77.96°. And then the ink contact angle exhibited a decrease when inorganic particles

Figure 8.  Printing performance of synthetic paper: a) TPU-N, b) TPU-S, c) TPU-C, d) TPU-E, e) TPU-Z, and f) TPU-T.

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3.6. Writing and Printing Performance

Figure 9.  Schematic diagram of water resistance for written words on synthetic paper under the circumstance of synthetic paper dipped into water at the beginning and after a 1 week obtained from: A1,A2) TPU-N, B1,B2) TPU-S, C1,C2) TPU-C, D1,D2) TPU-E, E1,E2) TPU-Z, and F1,F2) TPU-T.

filled in synthetic papers. The phenomenon could be probably explained as fillers were immobilized on the surface of the syn­ thetic paper, which increased the surface roughness and ink adsorption.

The writing and printing performances of synthetic papers are illustrated in Figures 7 and 8, respectively. The color of the word “SCIENCE” was vivid and clear. It is illus­ trated that all the samples had excellent writing and printing quality. However, in the writing process, the TPU-N synthetic paper was not written smoothly and the ink on the images can be wiped away easily. It is confirmed that TPU-N is not suitable for writing and printing. How­ ever, after loading with inorganic particles, synthetic paper showed smooth writing and printing performance. Moreover, the dispersion of inorganic particles and the micropore structure on the surface lead to a larger specific surface area for ink absorption, and the channel structures inside synthetic papers supported enor­ mous capacity for ink. In short, the syn­ thetic paper with inorganic particles have excellent writing and printing effects and it may be generalized in daily life.

3.7. Water Resistance Performance

Figures  9 and 10 show the water resistance effects of written and printed synthetic papers, respectively. As shown in Figure 9A1,9A2, the word written on TPU-N was blurred immediately when sample contacted water, and the word became vague after 1 week. From Figure 10A1,10A2, it can be seen that the ink diffused at once and the word almost disappeared. It is illustrated that the TPU-N synthetic paper showed poor water resistance performance. How­ ever, as shown in Figure 9(B1–F2 and Figure 10B1–F2, all of the synthetic papers with various kinds of inorganic particles performed well in water resist­ ance after 1 week. Obviously, the words were still clear and vivid, and the papers were not wrinkled and broken. As we all know, the process of ink adsorption relies not only on the porous structure but also on Van der Waals’ force between inorganic particles and ink molecules. Moreover, ink adsorption is a reversible physical process. Therefore, the ink on the surface tends to diffuse into the solution when its concentration is high, leading to words’ disappearance. On the contrary, owing to Figure 10. Schematic diagram of water resistance for printed words on synthetic paper under the circumstance of synthetic paper dipped into water at the beginning and after a 1 week was Van der Waals’ force, the synthetic paper obtained from: A1,A2) TPU-N, B1,B2) TPU-S, C1,C2) TPU-C, D1,D2) TPU-E, E1,E2) TPU-Z, and loaded with inorganic particles has good adhesion for ink molecules, resulting in F1,F2) TPU-T.

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good water resistance. In summary, inorganic particles can greatly improve the water resistance of synthetic paper.

4. Conclusions Polyurethane synthetic papers based on different inorganic fillers are successfully prepared using wet phase inversion process and inorganic mineral filling modification tech­ nique. The as-prepared synthetic papers have nano-sized pores, finger-like and spongy structures. Due to the agglom­ eration of inorganic particles, the active force between fillers and polymer decreased and the uneven stress in the filled papers appeared, resulting in a decrease of tensile strength and elongation at break. After diatomite, light calcium car­ bonate, eggshell, zeolite 13X, and transparent powder filling into synthetic papers, the fire-resistant property has a certain improvement. Especially when zeolite 13X powder is added into paper, the LOI value exhibits significant improvement from 19.75% to 31.8%. And this fact makes it promising for wide applications, such as important archives and wall­ paper. Compared with other filled synthetic papers, TPU-S exhibited higher water and oil adsorption capacity owing to unique diatom shell structure of diatomite powder. The ink contact angles of synthetic papers are less than 90°, demon­ strating that synthetic papers possessed good ink-affinity. The synthetic papers based on different inorganic fillers show excellent writing and printing qualities. In addition, synthetic papers show good water resistance and can meet the special requirement that general paper is not applicable for. How­ ever, DMF is not considered as a friendly solvent, which hin­ ders the popularization and application of synthetic paper. In the future work, we will devote our time to search for a more friendly solvent to prepare synthetic paper.

Acknowledgements This research was supported by the National Natural Science Foundation of China (21776067, 51573041, and 51773057), the Hunan Provincial Natural Science Foundation of China (14JJ5013), the Research Foundation of the Education Bureau of the Hunan Province, China (14B064 and 15A061), and the Planned Science and Technology Project of the Hunan Province, China (2015RS4044 and JC3112).

Conflict of Interest The authors declare no conflict of interest.

Keywords inorganic particle, polyurethane, synthetic paper, water and fire resistance Received: August 5, 2018 Revised: December 10, 2018 Published online:

Macromol. Mater. Eng. 2018, 1800473

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