Synthesis and Properties of Degradable Starch-based Waterborne Polyurethane Coatings

doi:10.21203/rs.3.rs-1607877/v1

Download PDF

Research Article

Synthesis and Properties of Degradable Starch-based Waterborne Polyurethane Coatings

https://doi.org/10.21203/rs.3.rs-1607877/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this latest preprint version

In this study, starch-based waterborne polyurethane (WPU) was successfully synthesized. The developed starch-based polyurethane was a key component with good degradation performance. It was further prepared to make the waterborne resin, which was self-emulsified and cured to form a film. Fourier transform infrared spectroscopy (FT-IR) showed that the necessary groups such as urethane bond and ether bond have been successfully incorporated into polyurethane. Moreover, transmission electron microscopy (TEM) and particle size distribution curve results showed that the synthesized starch-based WPU emulsion had small and uniform particle size and regular spherical particle morphology. In addition, scanning electron microscopy (SEM) detection showed that the surface of the coating film was smooth and had good adhesion. The starch-based WPU studied in this research had high thermal stability, good corrosion resistance, pollution-free to the environment. Moreover, the starch-based WPU is easy to be degraded, which has good application prospects.

Starch-based waterborne polyurethane

Self-emulsification

Corrosion resistance

Degradation performance

Coatings are a large class of materials coated and cured as a film on the surface of the substrate for purposes of decoration and protection. It is generally composed of film-forming substances dispersed (dissolved) in water or organic solvents. Traditionally, coatings dispersed with organic solvents generally contain high volatile organic compounds (VOCs), which results in difficult post-processing problems, such as air pollution, the greenhouse effect. The VOCs can exist in the environment for a long time and lead to great harm to the human body.

Starch is the main carrier of energy storage in most plant cells. It is formed by the dehydration and polymerization of glucose. Starch includes amylose and amylopectin. Starch comes from natural products, which is non-toxic, harmless, easy to degrade. Moreover, it has a wide range of sources and is renewable. At present, few reports on the water-based coatings prepared by starch or its derivatives as the main molecular skeleton can be found. Most of the starch and its derivatives are water-soluble products, which are difficult to be used for coatings. Therefore, the development of harmless, pollution-free, and easy-to-degrade coatings based on starch or its derivatives that do not cause cumulative environmental pollution is very important.

Waterborne polyurethane (WPU) is high molecular compound using water as dispersion medium, which is usually prepared by reacting binary or polybasic organic isocyanate with polyol compound. Due to its abrasion resistance, strong adhesion, good flexibility and other good properties, WPU has broad applications in the fields of coatings, adhesives, synthetic leather and industrial plastics[1–4]. The main resources of raw materials to synthesize WPU come from petrochemical products. However, the serious petroleum crisis influent a stable supplement of the raw materials for production of polyurethane[5]. Moreover, the traditional polyurethane is hard to be degraded in nature and is also difficult to recycle. Furthermore, the increasing quantity of traditional polyurethane deteriorate the environment. Due to increasing human awareness of environmental protection, concerning to their own health, and the strict requirements on emission of VOCs in environmental regulations, the applications of traditional polyurethane have been greatly limited [6–10]. Therefore, traditional polyurethane is supposed to be modified towards environmental-friendly and non-toxicity. A research hotspot, the modification of traditional polyurethane with less VOC emission while maintaining its inherent advantages, biodegradability for instance, came out recently. Using natural polymers such as lignin, starch and bark to modify or synthesize polyurethane is an effective way to endow it with biodegradability [11–13]. Among natural polymers, starch, which is a kind of polysaccharide compound, can not only be completely degraded as a natural renewable resource, but is also inexpensive compared with lignin and cellulose [14–16]. Moreover, starch can be used to directly synthesize polyurethane without any complicated processing. The application of starch and its derivatives in synthesis of modified polyurethane reduces VOC emission. Besides, it reduces the cost by reducing the consumption of petroleum-based polyols. As a result, the modification method is good for environment protection and sustainable development of resources [17–20].

Based on the above considerations, this experiment adopts solution polymerization method. Isophorone diisocyanate (IPDI) is used to introduce polyurethane segments and ether bonds are hydrophilic groups. Besides, water is added as dispersant. In sum, the traditional polyurethane is chemically modified into a self-emulsifying starch-based waterborne polyurethane. After synthesis, various properties are characterized. It is verified that the starch-based polyurethane is safe and nontoxic, easy to be degraded, pollution-free to the environment, and has good development prospects.

2.1 Materials

Potato starch (grade I, pure) was purchased from Henan Enmiao Food Co. Ltd (Henan, China); Isophorone diisocyanate (IPDI, industrial grade) was purchased from Shandong Moqian Chemical Reagent Factory (Shandong, China); Bisphenol, an epoxy resin (E-44, analytically pure) was provided by Shandong Yousuo chemical Reagent Factory (Shandong, China); acetic anhydride (analytically pure), glacial acetic acid (analytically pure), methanesulfonic acid (analytically pure) and polyethylene glycol (PEG, Mn = 200 and 2000, industrial grade) were provided by Kelong Chemical Co. Ltd (Chengdu, China). Prior to use, both acetate starch and PEG200 were dried in an oven at 60℃ for 10 h.

2.2 Synthesis of Acetate Starch

18g potato starch and 50 mL acetic acid were mixed and stirred in a three-necked flask equipped with a stirrer and a reflux condenser. 55.6 mL acetic anhydride was added as an acylating agent. Then, the reaction proceeded under mechanical stirring at the speed of 520 r/min for 15 minutes. Whereafter, 0.1mL methanesulfonic acid was immediately added as a catalyst to accelerate the reaction into the mixture and kept stirring at 75℃ for 4 hours. When the reaction was finished, the product was poured into distilled water and washed repeatedly until the pH was neutral. Finally, the product was filtered and dried to constant weight in a vacuum drying oven at 50℃. The scheme of the preparation process for acetate starch was shown in scheme 1.

2.3 Preparation of Starch-based WPU Emulsion

IPDI and PEG 200 (-NCO:-OH molar ratio was 2:1) and 20mL amount of butanone was added into a three-necked flask, which was equipped with a stirrer and a reflux condenser tube, mechanically stirred at 70℃ for 2 hours at a rotating speed of 330 r/min. Then, 6.37g acetate starch, 8.2g epoxy resin E-44 and 3.6g PEG2000 were sequentially added, the temperature was raised to 75℃ for 4 hours. Finally, the temperature was lowered to 50℃, a proper amount of distilled water was added with rapid stirring and starch-based polyurethane emulsion was obtained. The scheme of the preparation process for starch-based polyurethane emulsion was shown in scheme 2.

2.4 Preparation of Coating Film

The synthesized starch-based WPU emulsion and curing agent high imino methylated melamine resin were mixed according to a certain proportion. After the mixture was evenly mixed, the mixture was sprayed on a 90×120×5 mm glass plate and 50×120×0.2 mm and 50×100×0.5 mm tinplate. In a vacuum drying oven, the temperature of 160℃ was maintained for 20 min to obtain a coating film for further testing.

Fourier transform infrared spectroscopy (FT-IR, Bruker-VECTOR-22) provided by German Bruker Instrument Company was used to obtain the infrared spectrogram. The starch-based WPU emulsion was dropped on KBr tabletting and 2 mg coated film sample was ground and tabletted together with KBr and tested. The scanning range was 4500 − 400 cm^− 1 and 16 scanning times were used. Transmission electron microscopy (TEM) images were obtained with a JEM-200CX Microscope at an accelerating voltage of 100 kV, and all samples were stained with 0.2 wt.% phosphortungstic acid hydrate before observation. The particle size and its distribution were measured by Nano-ZS90 Zeta potentiometer. The emulsion was diluted to the ratio of 1:100 and the testing temperature was 25℃. Thermogravimetric analyzer (NETZSCH STA 449 F3 TGA/DSC analyzer) was used to measure thermal weight loss, heat absorption and heat release of the starch-based waterborne polyurethane emulsion. The parameters were set as follows: under N₂ atmosphere, the temperature was raised from 30℃ to 600℃ with the heating rate of 10℃/min. The stability was measured using a HC-3018 high speed centrifuge (ZONKIA, China) at a speed of 3000 r/min for 15 min. If no precipitate was observed after centrifugation, the sample was considered stable for 6 months. Scanning electron microscope (SEM) was used to observe the surface morphology of the coating. Before observation, the emulsion was solidified into a film and sprayed with gold. Potential of polarization curve of the coated membrane was measured in an electrochemical test system composed of PARSTAT2273 potentiostat and microcomputer. In this test, a three-electrode system was adopted, NaCl solution was used as electrolyte, saturated calomel electrode was used as reference electrode, platinum electrode was used as auxiliary electrode, scanning range of test voltage was − 1V ~ 1.0V, the scanning speed was 2 mV/s, and the area of the test sample was 1 cm². Before testing, the sample should be immersed in solution for 30 minutes.

In order to obtain the degradation performance of the coating film, the solidified starch-based WPU films were weighed and recorded. Then, they were placed in a 50 mL hydrothermal reaction kettle, and 25 mL distilled water was added before the reaction kettle was sealed. The sealed reaction kettle was placed in a vacuum drying oven at 150℃, and the films were taken out after reacting for 1 hour, 2 hours, 3 hours and 4 hours, respectively. Then, the films were cooled to normal temperature. Finally, the films were dried at 70℃ and weighed again. Finally, surface morphology of the films was observed.

4.1 Chemical characterization and Microscopic observation

Figure 1 shows the FT-IR spectra of raw starch, starch-based WPU emulsion and starch-based WPU coating film after emulsion curing. The peaks at 1695 cm^-1 and 1226 cm^-1 represent stretching vibration peaks of C = O and C-O in urethane bond, respectively. The peaks at 3442 cm^-1 and 1538 cm^-1 are attributed to the absorption peak of expansion and vibration and absorption peak of deformation vibration of N-H bond, which indicates that -NH-COO- have been formed [21]. The characteristic peak of hydroxyl groups (-OH) appears at 3400–3640 cm^-1, and the peak intensity is gradually weakened. When the coating film is prepared, the absorption peak of -OH is almost disappeared, indicating that the reaction product introduces starch-based structure and almost all -OH in starch are participated in reaction and completely consumed [22]. Therefore, the FT-IR spectra shows that starch-based WPU has been successfully synthesized.

Figure 2 shows that it can barely find irregular micro-phase areas, such as micro cracks and spots, on the surface of the cured coating film. The surface of the coating film is relatively uniform and smooth, indicating that the shrinkage of the coating film is not visibly found during the curing process. The addition of epoxy E-44 can better improve the shrinkage problem of starch-based WPU during the curing process.

4.2 Particle size and distribution of the starch-based WPU emulsion

The particle size and particle size distribution of starch-base WPU emulsion are shown in Fig. 3. The results show that a narrow particle size distribution of the emulsion is obtained. The average particle size is about 130 nm, which is very small. Moreover, the particle size distribution is uniform, which is consistent with the TEM observation in Fig. 4.

Figure 4 shows TEM images of the starch-based WPU emulsion. It is observed that the particle size distribution of the synthesized starch-based WPU is relatively uniform, and the particle shape is close to relatively regular spherical, which presents a stable dispersion state. The reason is that in the emulsification process of the modified polyurethane resin, hydrophobic segments are gathered to form aggregated particles. Hydrophobic segments are distributed inside the particles and hydrophilic segments are located on the surface of the particles. Due to the effects of electrostatic force and surface tension, the particles are in spherical shape and uniformly distributed [23]. Therefore, the particles can be dispersed in water, forming a stable dispersed liquid with relatively small particle size.

4.3 Thermal properties

The thermogravimetric analysis curve of starch-based WPU film is shown in Fig. 5, and the corresponding data are summarized in Table 1. An obvious change of the thermogravimetric curve of starch-based WPU coating film can be found. From Fig. 4, the thermal decomposition curve of starch-based WPU coating film is mainly in two stages: the first stage is at 100–380℃, which is mainly resulted from the degradation of the molecular chain of the coating resin at high temperature. This phenomenon can be attributed to the chain break of urethane group and polyester, and the decomposition of starch-based WPU to the products of diisocyanate and polyo [24]; the second stage is at 380–600℃, which is mainly resulted from the decomposition of diisocyanate containing hard segment structure to generate imide and isocyanate, and the decomposition of polyol with soft segment structure to generate olefin[25]. It can be seen from thermal weight loss rate curve that the extreme value of weight loss rate is appeared at about 380℃. It shows that the starch-based WPU coating prepared in this experiment has excellent thermal stability, which can ensure its application in high temperature environment.

Table 1

Thermogravimetric data of starch-based WPU coating.
Sample	T_5%℃	T_18%℃	T_40%℃	T_80%℃
PS-WPU	150	330	380	600
Note: T_X%℃ indicates the temperature when the sample loses weight by X%

4.4 The corrosion resistance of the starch-based WPU

As shown in Fig. 6, A represents the polarization curve of the starch-based WPU cured film and B represents the polarization curve of the starch-free WPU coating film. From the polarization curve, it shows that when the self-corrosion potential is 0.06 V, the corrosion current density of the starch-based WPU coating sample is 2.48×10^− 3 A/cm², while the self-corrosion potential of the starch-free WPU coating is -0.5V. The corrosion current density of the coating sample is 8.23×10^− 3 A/cm². It is revealed that the starch-based WPU coating has higher self-corrosion potential, lower corrosion rate and lower corrosion tendency compared with that of starch-free WPU. This is possibly attributed to the addition of starch acetate bonding onto the backbone of starch-based WPU, where both acetyl and ester groups in the monomer are attached [26–27]. Therefore, the introduction of starch acetate groups is conducive to improve the corrosion resistance of the starch-based WPU coatings.

4.5 Degradation performance of the starch-based WPU coating film

Figure 7 shows high-temperature (150℃) degradation results of starch-based WPU coating. It is observed that holes and cracks of starch-based WPU coating are gradually developed respect to degradation time, indicating that the coatings can be degraded in high-temperature distilled water condition[28–30]. As shown in Fig. 7 and Table 2 that, in the early stage of degradation reaction, the degradation rates of starch-based WPU coating are slowly increased. With the increase of time, the slopes of curves are gradually increased, indicating that the degradation rate is accelerated. The degradation rate reaches 52.8% after 4 hours degradation. Therefore, starch-based WPU coating is easier to be degraded. It is verified that the addition of starch is beneficial to the degradation of WPU.

Table 2

Degradation rate of starch-based WPU in distilled water at 150℃.
Time	1h	2h	3h	4h
Samples	4.1%	12.7%	31.3%	52.8%

In this study, starch-based WPU was successfully synthesized. The developed starch-based polyurethane was a key component with good degradation performance. It was further prepared to make the waterborne resin, which was self-emulsified and cured to form a film. Fourier transform infrared spectroscopy (FT-IR) showed that starch was successfully incorporated into polyurethane. Transmission electron microscopy (TEM) and particle size distribution curve analysis showed that the synthesized starch-based WPU emulsion had small particle size and uniform particle size distribution. The particle shape was close to regular spherical, showing a stable dispersion state. Moreover, with increasing molecular weight of polyethylene glycol, the particle size of the emulsion was smaller. The thermogravimetric (TG) curve, and scanning electron microscope (SEM) analysis showed that the cured starch-based WPU coating has a good thermal stability and high temperature resistance. The starch-based WPU coating had a uniform and smooth surface. Polarization curves showed that the coating film had good corrosion resistance. Through degradation of the coating film, it was shown that, with increasing degradation temperature and degradation time, the coating film containing starch group was degraded more completely. Therefore, the starch-based WPU studied in this research had high thermal stability, good corrosion resistance, pollution-free to the environment. Moreover, the starch-based WPU is easy to be degraded, which has good application prospects.

Acknowledgments

This work was financially supported by Sichuan Science and Technology Program (No. 2019JDTD0024 & No. 2019ZHCG0048) and “Xihua Scholars” program of Xihua University.

Conflicts of Interest: The author declares no conflict of interest.

Liu H (2012) Cellulose nanocrystal/silver nanoparticle composites as bifunctional nanofillers within waterborne polyurethane. ACS Appl Mater Interfaces 4(5):2413–2419
Wang Y (2019) Antimicrobial Waterborne Polyurethanes Based on Quaternary Ammonium Compounds. Ind Eng Chem Res 59(1):458–463
Xuan Yin YL, Jing Z (2017) Synthesis and Characterization of Halogen-Free Flame Retardant Two-Component Waterborne Polyurethane by Different Modification. Ind Eng Chem Res 56:1791–1802
Zhang Y (2018) Antibacterial and Biocompatible Cross-Linked Waterborne Polyurethanes Containing Gemini Quaternary Ammonium Salts. Biomacromolecules 19(2):279–287
Zhou C (2015) Waterborne Polyurethanes with Tunable Fluorescence and Room-Temperature Phosphorescence. ACS Appl Mater Interfaces 7(31):17209–17216
Zhang L, Zhang H, Guo J (2012) Synthesis and Properties of UV-Curable Polyester-Based Waterborne Polyurethane/Functionalized Silica Composites and Morphology of Their Nanostructured Films. Ind Eng Chem Res 51(25):8434–8441
Zhang J et al (2017) Novel Waterborne UV-Curable Hyperbranched Polyurethane Acrylate/Silica with Good Printability and Rheological Properties Applicable to Flexographic Ink. ACS Omega 2(11):7546–7558
Zhang M, Zhao F, Luo Y (2019) Self-Healing Mechanism of Microcracks on Waterborne Polyurethane with Tunable Disulfide Bond Contents. ACS Omega 4(1):1703–1714
Zhao J et al (2014) Synthesis of a Waterborne Polyurethane-Fluorinated Emulsion and Its Hydrophobic Properties of Coating Films. Ind Eng Chem Res 53(49):19257–19264
Zheng H et al (2019) Robust, Transparent, and Superhydrophobic Coating Fabricated with Waterborne Polyurethane and Inorganic Nanoparticle Composites. Ind Eng Chem Res 58(19):8050–8060
Xie DY et al (2016) Roles of Soft Segment Length in Structure and Property of Soy Protein Isolate/Waterborne Polyurethane Blend Films. Ind Eng Chem Res 55(5):1229–1235
Xu Y et al (2018) Gradient Structure Design of Flexible Waterborne Polyurethane Conductive Films for Ultraefficient Electromagnetic Shielding with Low Reflection Characteristic. ACS Appl Mater Interfaces 10(22):19143–19152
Zhang Y et al (2011) Tough Thermoplastic Starch Modified with Polyurethane Microparticles: Effects of Molecular Weight of Soft Segments in Polyurethane. Ind Eng Chem Res 50(4):2111–2116
Shao J et al (2019) Preparation and Properties of Hydrophilic Rosin-Based Aromatic Polyurethane Microspheres. ACS Omega 4(2):2493–2499
Sirvio JA, Heiskanen JP (2019) Carbamation of Starch with Amine Using Dimethyl Carbonate as Coupling Agent. ACS Omega 4(13):15702–15710
Wu Q et al (2011) ). Tough Thermoplastic Starch Modified with Polyurethane Microparticles: The Effects of Processing Temperatures. Ind Eng Chem Res 50(4):2008–2014
Leng Y et al (2011) Hydrophobic Thermoplastic Starches Modified with Polyester-Based Polyurethane Microparticles: Effects of Various Diisocyanates. Ind Eng Chem Res 50(19):11130–11135
Pathan S, Ahmad S (2013) s-Triazine Ring-Modified Waterborne Alkyd: Synthesis, Characterization, Antibacterial, and Electrochemical Corrosion Studies. ACS Sustain Chem Eng 1(10):1246–1257
Runan, Gong et al (2020) High-Performance Waterborne Polyurethane Coating Based on a Blocked Isocyanate with Cellulose Nanocrystals (CNC) as the Polyol. ACS Appl Polym Mater 2:385–393
Wang Y et al (2019) Core–Shell Starch Nanoparticles and Their Toughening of Polylactide. Ind Eng Chem Res 57(39):13048–13054
Liang H et al (2019) Tailoring the Performance of Vegetable Oil-Based Waterborne Polyurethanes through Incorporation of Rigid Cyclic Rings into Soft Polymer Networks. ACS Sustain Chem Eng 8(2):914–925
Lingyuan, Liao et al (2016) Effects of Surface Structure and Morphology of Nanoclays on the Properties of Jatropha Curcas Oil-Based Waterborne Polyurethane/ Clay Nanocomposites, vol 55. Industrial & Engineering Chemistry Research, pp 11689–11699
Wen JG et al (2019) Improvement of Corrosion Resistance of Waterborne Polyurethane Coatings by Covalent and Noncovalent Grafted Graphene Oxide Nanosheets. ACS Omega, 2019. 4(23): p. 20265–20274
Gurunathan T, Chung JS (2016) Physicochemical Properties of Amino–Silane-Terminated Vegetable Oil-Based Waterborne Polyurethane Nanocomposites. ACS Sustain Chem Eng 4(9):4645–4653
Mingjie Guo XJ, Heqing Fu (2018) &. Preparation and Study of Novel Modified [(1-x)MnO2xMWCNTs]/ Waterborne Polyurethane Composites with Microwave Absorption Properties. Industrial & Engineering Chemistry Research, 2018. 57: p. 13406 – 13416
Fang Y et al (2018) Thermal-Driven Self-Healing and Recyclable Waterborne Polyurethane Films Based on Reversible Covalent Interaction. ACS Sustain Chem Eng 6(11):14490–14500
Gaddam SK, Kutcherlapati SNR, Palanisamy A (2017) Self-Cross-Linkable Anionic Waterborne Polyurethane–Silanol Dispersions from Cottonseed-Oil-Based Phosphorylated Polyol as Ionic Soft Segment. ACS Sustain Chem Eng 5(8):6447–6455
Hanbin Liu W, Huang, Huacui Xiang (2020) Flexible and Degradable Multimodal Sensor Fabricated by Transferring Laser-Induced Porous Carbon on Starch Film. ACS Sustainable Chem Eng 8:527–533
Ma Z et al (2020) Synthesis of CO2-Derived, Siloxane-Functionalized Poly(ether carbonate)s and Waterborne Polyurethanes. Ind Eng Chem Res 59(7):3044–3051
Rani P, Srivastava R (2019) Starch Coated Silica Nanospheres Parenting the Growth of Trimodal Porous Zeolites for Catalysis Involving Large Molecules. ACS Sustain Chem Eng 7(11):9822–9833

Download PDF

Version 1

posted

You are reading this latest preprint version

Synthesis and Properties of Degradable Starch-based Waterborne Polyurethane Coatings

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1 Materials

2.2 Synthesis of Acetate Starch

2.3 Preparation of Starch-based WPU Emulsion

2.4 Preparation of Coating Film

3. Characterizations

4. Results And Discussion

4.1 Chemical characterization and Microscopic observation

4.2 Particle size and distribution of the starch-based WPU emulsion

4.3 Thermal properties

4.4 The corrosion resistance of the starch-based WPU

4.5 Degradation performance of the starch-based WPU coating film

5. Conclusion

Declarations

References

Status:

Version 1