Introduction
Cationic polymer latex nanoparticles are an important class of materials and have been widely employed in diverse applications in paper manufacturing for both wet-end and surface applications, cosmetics, food packing and biomedical applications as drug carriers [[1], [2], [3], [4], [5], [6], [7]]. In this sense, cationic starches are an alternative to produce cationic polymeric nanoparticles with a wide range of properties, such as positive charge, colloidal stability at low pH, porosity and printability for food packaging and paper coating applications [5,[7], [8], [9]]. Further, specific to the positive charge, one of the major advantages of cationic starch is their antimicrobial characteristic [2,4,6,8,9] and their applications for printing on paper, or printability [5].
Starch is one of the most abundant linear polysaccharides present in various plants and produced in large scale from rice, potato, cassava and corn [10,11]. It is biodegradable, biocompatible, non-toxic and hydrophilic, formed from anhydroglucose units, α(1–4) and α(1–6) glycosidic bonds [10,11]. The linear portion of starch, known as amylose, is composed mainly of α(1–4) linkages while the α(1–6) bonds give rise to a branched structure known as amylopectin. The ratio of α(1–4) linkages to α(1–6) linkages determines the type of starch and is responsible for specific properties like, water solubility and crystallinity [10,11]. Native starch has to be modified by chemical or physical methods to obtain the required properties for paper and food industry [7,10]. Cationic starches can be produced by reacting starch with compounds containing tertiary or quaternary ammonium, amino groups or cationic monomers, yielding a positive charge [[10], [11], [12]] and are available in the market in various grades, depending on their specific application [13].
Cationic starch and its derivatives have been employed extensively in the production of sustainable polymeric latex nanoparticles employing different vinyl monomers and initiators to produce renewable materials in paper coating as binders and sizing agents, with properties that combine the advantages of starch and synthetic polymer [7,8,[14], [15], [16], [17], [18]]. One of the main functions of cationic starch graft polymer latex in surface sizing is to improve paper strength and surface porosity for cost competitiveness, and to achieve better interaction between the cationic moieties of starch with negatively charged ink particles for improvements to printability [5,18,21,22]. On the other hand, the use of cationic starch as a coating binder is limited by its sensitivity to water. Previous studies have proven that starch-graft-copolymers have an advantage over the simple mixtures of synthetic latex or modified starch [5,7,8,[17], [18], [19]]. For this purpose, cationic starch needs to be modified chemically, enzymatically or physically to obtain reduced molar mass before monomer addition in order to improve the stability of polymer nanoparticles [8,[14], [15], [16], [17], [18], [19], [20], [21], [22], [23]].
An industrial approach is based on the in-situ treatment of cationic starch by amylases for conversion of starches into oligosaccharides through the cleavage of glycosidic bonds [8,[15], [16], [17], [18], [19], [20], [21], [22], [23]]. Earlier investigations have shown that enzymatic treatment is a better method in preparing polymeric nanoparticles stabilized by starch for paper coating [8,[15], [16], [17], [18], [19], [20]] over other grafting techniques. Brockmeyer, et al. [15] described the use of a α-amylase to cleavage of glycosidic bonds of starch with substitution degree of 0.045 mol mol−1 at 85 °C for 30 min. Cheng et al. [8,[18], [19], [20]] reported about degradation time of cassava starch by α-amylase at 80 °C and grafting efficiency in an emulsion polymerization of styrene, MMA and BA.
Normally the solid content in an industrial formulation of enzymatically hydrolyzed cationic starch to produce polymer latex for paper application is between 25 and 30 % due the limitations in viscosity and stability of the latex. The presence of starch-free, low degrees of grafting level, sensitivity to water and the use of enzymes can limit the viability of polymer latex for paper-coating applications [5,24,25]. In this sense, various drawbacks were still present, such as latex instability, cytotoxicity of polymer films and the presence of enzymes even in very small residual quantities. Therefore, it is desirable to understand the cytotoxicity of polymer films and CCS employed for paper application to reduce the amount of chemicals used in the paper industry and in surface treatment processes specifically.
Recently, Li et al. [24] reported that polymer latexes used in paper coatings may cause toxic effects in cells or organisms due the possible migration from paper packaging to food. For this reason, the cytotoxicity of cationic nanoparticles has been studied in this area for many risks to food contamination and other purposes [26]. However, given the industrial and scientific importance of polymeric nanoparticles stabilized by cationic starch for paper coating and the lack of reported data about the cytotoxicity of the polymer latex films, CCS and enzymes, an investigation into the CCS content was conducted.
In this work, the effect of enzymatic cationic starch on the properties of starch-graft-poly(n-butyl acrylate-co-methyl methacrylate) latexes and films, produced by surfactant-free emulsion polymerization was studied. The purpose of this work was to understand the effect of enzymatically degraded CCS content (from 0 up to 20 wt.%) on the colloidal properties of the latexes and on the cytotoxicity of polymer latex films in human keratinocytes cells (HaCaT) for 24 and 96 h. Cytotoxicity information is helpful for industrial application, such as paper coating to predict the cytotoxic effect of newly developed materials. Poly(BA/MMA-g-CCS) latexes were employed as sizing agents for paper, and the surface wettability was evaluated by Cobb60, Hercules sizing test and contact angle analysis. This work was developed in collaboration with BASF S.A., as an effective strategy for the use of starch-graft-polymer as binders and sizing agents for the paper and packing industry.
Section snippets
Materials
Methyl methacrylate (MMA) and n-butyl acrylate (BA) were supplied by BASF SA (Brazil) and were washed with NaOH solution (2.5 wt.%) to remove any traces of inhibitor and stored at −4 °C prior to use. Ferrous ammonium sulfate hexahydrate was obtained from Millipore-Sigma and hydrogen peroxide (HPO, 35 %) was obtained from Acros Organics. Cationic cassava starch (CCS, Amylofax® T15) was provided by Avebe, purified by washing with ethanol several times and then dried in an air circulation oven at
CCS properties
In order to understand the parameters controlling the polymeric nanoparticles and extent of enzymatic degradation by α-amylase enzyme, the properties of CCS used in this work were determined and the results are summarized in Table 2. It can be observed that the medium degree of substitution (DS) obtained by Kjeldahl’s method was 0.293 and the pH of aqueous solution at 30 % wt./v was 6.6. The results of DS obtained by elemental analysis and 1H NMR are also listed in Table 2 and are in agreement
Conclusion
Stable cationic polymer latexes with different amounts of hydrolyzed cationic cassava starch (CCS) were successfully obtained by one-pot semi-batch emulsion polymerization. The latexes can be applied in several industrial applications including paper coatings. The grafting percentage decreased as the concentration of CCS increased. The optimal CCS concentrations that maximized not only particle size, but also water resistance and stability, were 7 and 10 wt.%, which are directly related to the
CRediT authorship contribution statement
Lina D.A. Rodrigues: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing - original draft. Carolina R. Hurtado: Formal analysis, Methodology. Erenilda F. Macedo: Formal analysis, Methodology. Dayane B. Tada: Funding acquisition, Project administration, Validation. Lília M. Guerrini: Data curation, Supervision, Visualization, Writing - review & editing. Maurício P. Oliveira: Funding acquisition, Project administration, Resources, Software, Supervision,
Declaration of Competing Interest
The author declares that there is no conflict of interest.
Acknowledgements
The authors gratefully acknowledge the financial support of the Sao Paulo Research Foundation (FAPESP, grant 2018/12469-7 and 2017/01697-6), National Council for Scientific and Technological Development (CNPq, grant 314898/2018-2). We also thank BASF Brazil for technical support and for the reagents used in this work.
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