Green synthesis and properties of an epoxy-modified oxidized starch-grafted styrene-acrylate emulsion
Graphical abstract
Introduction
Poly(styrene-co-acrylate) is one of the most familiar emulsion copolymers. Its specific properties, including good film formation, gloss, transparency, and mechanical properties, make it significant in many fields [1], [2], [3], [4]. Particles with core-shell morphology have some advantages over pure polymer particle and can be designed for versatile properties. Therefore, researchers have developed multiple approaches to improve poly(styrene-co-acrylate) emulsions. Tan synthesized a poly(styrene-co-butyl acrylate) emulsion via a structured core-shell particle design, finding that the polymer exhibited high cohesive and adhesive strength when the core-shell ratio was 1:1.5 [5]. Eren and coworkers developed novel core-shell-style styrene acrylic polymers to reduce the N-hydroxymethyl acrylamide (NMA) content and formaldehyde formation from pigment printing [6]. However, a series of shortcomings, such as poor water resistance, poor heat resistance, low pencil hardness, and high cost, still greatly limited the application of the materials [7]. In recent years, core-shell-style styrene acrylic polymers have been improved by changing the ratio between butyl acrylate (BA) and styrene (St) monomers or the order of the two monomers in the shell polymerization process [8], [9], [10], as well as by alternating the surfactant and initiator in the polymerization process [11], [12]. Additionally, core-shell-style styrene acrylic polymers can be modified by functional materials. Chen mixed PS-Ag conductive particles with particles of poly(styrene-co-butyl acrylate) and obtained large-scale anisotropic conductive films [13]. In addition, chemical modification of styrene-acrylic emulsions is a common method of improving their performance. By adding acrylonitrile to the shell material, acrylonitrile-styrene-acrylate (ASA) latexes were prepared to obtain a thicker latex shell structure, which improved the mechanical properties of the emulsion film [14]. A silane-modified styrene-acrylic emulsion improved the waterproof performance and softening coefficient of FGD gypsum products [3]. However, a relatively low level of corrosion protection, especially for compositions without special protective ingredients, is the main drawback [15], [16], [17]. Coatings are a good barrier against the permeation of water, oxygen, and other corrosive species but can be quite weak due to various factors, such as residual hydrophilic components and a low cross-linking degree, which are often related to the use of a purely physical curing mechanism.
Starch is a carbohydrate obtained from plant photosynthesis and has the advantages of biocompatibility, biodegradability, low cost and no toxicity [18], [19], [20]. Starch has received extensive attention in the food [21], biomedical [22], wastewater treatment [23], [24], paper [25], and textile industries [26] because of its unique biological and adsorption properties. However, native starch has low shear stress resistance, low decomposition, high retrogradation, and syneresis properties. Moreover, its large molecular weight and condensation limit the application of native starch in many fields [27], [28]. Extensive work on starch modification has been implemented to overcome these inherent defects and achieve innovative starch applications; the techniques included physical processes [29], [30], [31] chemical modifications [32], [33], [34], [35] enzymatic [36], [37] and biotechnological approaches [38] and combinations thereof. Some properties, including gelatinization temperature [39], thermal stability [40], rheological properties, paste clarity [41], and mechanical strength [42], can be improved via chemical modification. Starch oxidation is an important part of the chemical modification of starch. Researchers usually oxidize starch and apply the oxidized starch to other polymers. The introduction of grafted starch can improve the elasticity of PBA chains [43]. Meshram grafted St and MMA/BA onto starch molecules and observed a higher tensile strength and lower elongation at break when the material was used in cotton yarn [44]. Therefore, the synthesis of starch graft copolymers to combine the advantages of natural and synthetic polymers has aroused interest from many researchers. Gaborieau proved that the application properties of starch graft copolymers were better than those of common latex [45], [46]. Cheng succeeded in preparing oxidized starch-graft-poly-(styrene-butyl acrylate) and proved that the prepared polymer colloid had strong stability, fine particles and strong permeability [46], [47], [48]. Disappointingly, its adhesion and water resistance did not reach elevated expectations.
In nature, epoxy resin is a kind of polymer that has advantages such as good thermal characteristics, excellent adhesion, and good chemical and corrosion resistance [49], [50], [51]. Deng and Djukic blended epoxy resin with a polymer to study the possible mechanism of their interfacial adhesion [51]. Zhang found that epoxy helped amine-containing benzoxazine-based homopolymers overcome the conversion limitation and promoted the curing rate [52]. Epoxy-modified styrene-acrylic emulsions have the dual properties of an epoxy resin and styrene-acrylic emulsion, where the epoxy resin is used as a cross-linking agent. Epoxy resin contains polar groups, including hydroxyl groups and epoxy groups, which easily form secondary bonds, hydrogen bonds and main valence bonds with many surface substances. Moreover, epoxy and hydroxyl groups can react with functional groups on other compounds (such as amino groups, hydroxyl groups and carboxyl groups) to form a network structure [53], [54]. Thus, styrene-acrylic emulsions modified with epoxy resin possess strength, corrosion resistance and adhesion. Such emulsions have been widely applied to improve viscosity, mixture and coating properties [55], [56]. However, due to the high viscosity of epoxy resin, an excessive addition amount causes the polymer emulsion to gel. Additionally, organic solvents used as dispersion media cause residual organic solvent and environmental pollution.
In this work, the polymerization monomer participating in the reaction was used as a dispersion medium for the epoxy resin, avoiding environmental problems caused by volatilization of the organic solvent. E-g-OS-P (BA-St) latexes, which integrate the advantages of epoxy, oxidized starch and styrene-acrylate emulsions, were synthesized via soap-free core-shell emulsion polymerization. This approach has not been previously researched. The effect of the amount of epoxy resin on the properties of E-g-OS-P (BA-St) latex was studied. The obtained latexes are expected to have excellent chemical properties, thermal stability and mechanical properties. The introduced oxidized starch is biodegradable and ecofriendly. Oxidized starch can not only improve the dispersion stability of emulsions but also reduce the cost of emulsion production. Additionally, the mechanisms of polymerization and film formation were analysed, which can provide a theoretical reference for coating applications.
Section snippets
Materials
Oxidized starch was provided by Shangdong Jincheng Co., Ltd. Styrene (St, AR) and butyl acrylate (BA, AR) were purchased from Shanghai Chemical Reagent Co. and were purified by distillation under reduced pressure. Epoxy resin was supplied by Xinhua Resin Manufacturing, Shanghai Coatings Co. Hydrogen peroxide (H2O2, 30%) was obtained from Tianjin Damao Chemical Reagent Factory, China. Deionized water was prepared in our laboratory.
Preparation of oxidized starch
The corn starch (30 g) and deionized water (120 g) were weighed
Conversion and gel fraction
The conversion and gel fraction percentage were determined by a gravimetric technique.
A few drops of hydroquinone aqueous solution were added, and a certain mass (W1) of emulsion was weighed into a dry culture dish. The mass of the dry culture dish was labelled W0. Afterwards, the sample was placed in an oven at 120 °C and dried to constant weight (W2).
The monomer conversion was calculated as follows [57], [58], [59]:where W1 is the weight of the wet sample, W2 represents
Physical-chemical characterization of the emulsions
Kan determined the storage stability of emulsions by adding electrolyte [63]. If latex is sensitive to the electrolyte, such as NaCl and Na2SO4, a certain amount of coagulum will appear. All the as-prepared latexes were resistant to NaCl and Na2SO4, even at their saturated concentrations. Moreover, there was no condensation when the latex was melted after being stored at −5 °C. No condensation occurred during storage for 6 months or during 1 h of centrifugation at a speed of approximately
Conclusion
E-g-OS-P (BA-St) latexes were synthesized via soap-free core-shell emulsion polymerization without adding any solvent that could cause environmental problems. The addition of epoxy resin had an important effect on the morphology of E-g-OS-P (BA-St) latexes and the mechanical properties of the films. In addition, epoxy resin increased the thermal stability and Tg of the films. When the epoxy resin content accounted for 10% of the total amount of the polymerized monomers, the emulsion exhibited
CRediT authorship contribution statement
Shaoxiang Li: Conceptualization, Writing - review & editing, Supervision. Jiaping Wang: Software, Writing - original draft, Formal analysis. Wenjuan Qu: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Supervision, Funding acquisition. Jiaji Cheng: Investigation, Writing - review & editing, Funding acquisition. Yunna Lei: Software. Dong Wang: Methodology, Formal analysis. Feng Zhang: Investigation, Funding acquisition.
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgements
This work was financially supported by the Natural Science Foundation of Shandong Province, China (No. ZR2016EMB20 and ZR2018MEE034), the National Natural Science Foundation of China (Grant No. 51806113) and the Postdoctoral Applied Research Program Fund of Qingdao (No. 2016204).
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