Controlled biodegradability of functionalized thermoplastic starch based materials

Highlights

• Appropriately plasticized starch-based materials with optimized mechanical properties were prepared by 2-step processing.

• Biodegradation rate is significantly retarded as a function of the starch chemical modification, especially acetylation.

• Final material homogeneity is an important factor affecting the biodegradation rate.

• The presence of acetylated starch does not influence the quality of the soil substrate.

Abstract

The present work focuses on the preparation and characterization of biodegradable materials based on plasticized and chemically modified starch. During the experiments, the morphology, properties and biodegradability of the starch materials were influenced by the functionalization (acetylation, propionation) of the starch, different processing procedures and the use of plasticizers. The starch materials were prepared by either solution casting or melt mixing or by the combination of the two procedures, which strongly affected the degree of the material plasticization, its homogeneity, and the final morphology. FTIR spectroscopy qualitatively proved the esterification of starch by the intensity of signals associated with esters groups. The thermogravimetric analysis confirmed a gradual reduction in the water content proportional to the acetylation of starches, as well as noticeable changes in their thermal properties at the higher degree of substitution (DS). Signal intensities and weight losses derived from FTIR and TGA, respectively, were well correlated with DS. The morphological changes of the polysaccharide materials were visualized by microscopy techniques. The biodegradation rate of prepared materials was expressed as the percentage of carbon mineralization and was significantly retarded as a function of the starch acetylation. A remarkable effect of the material processing, the presence of plasticizers, and the propionation of the starch on biodegradation has been found.

Introduction

During the last decades, many studies have been focused on biodegradable natural polymers, such as starch, cellulose and many others, with the aim to create new materials with desired properties. A significant part of all bio-based plastics is derived from or contain starch. The reasons are low cost, total biodegradability and low overall carbon footprint. In addition, starch is produced in excess with regard to the current market needs. Consequently, starch and other polysaccharides either in their native form or after a suitable modification, are used as raw materials for the production of many bioplastics [1,2].

Wheat starch forms two different types of granules: so-called “A-starch” and “B-starch”. Wheat A-starch contains larger (more than 25 μm) particles with a lenticular shape. Wheat B-starch consists of smaller spherical particles with the average diameter of about 5 μm [2]. At the same time, the B-starch has higher concentrations of proteins, lipids, and pentosans, lower amylose content and exhibits higher gelatinization temperatures. Processing of this material (filtration, drying, etc.), as well as food applications, are limited due to the smaller size of these granules. Therefore it is considered a low-quality by-product in the starch industry and its application possibilities are being explored [1].

The materials made of plasticized starch proved to be useful for a number of applications due to their mechanical properties, effortless processing, fast biodegradation, low ecotoxicity, and renewability as well as due to their low price [1,3]. In order to transform a native powder starch into a thermoplastic polymer, it is necessary to break up its granular structure. The bulk thermoplastic starch (TPS) is formed during processing under a combined use of shear forces, temperature, and time, usually in the presence of appropriate additives [4]. Unfortunately, TPS shows high water absorption and rather low mechanical performance compared to conventional synthetic polymers [5].

The transformation of semicrystalline starch granules into a bulk thermoplastic matrix can also be achieved chemically through the acetylation process associated with the modification of initially highly hydrophilic chains by substituting some hydroxyl groups [6]. Since the water sorption in starch also depends on its polarity, the esterification of the starch hydroxyl groups may result in a decrease of hydrophilicity [7,8]. A reduction of the water content can also be accomplished by other methods, among others by carefully selecting the plasticizers used [9].

The chemical modification of starch is usually done in solution because of the high molecular weight of amylopectin and therefore high viscosity of its melt. If a reaction in melt is required, the molecular weight of starch must be reduced by partial hydrolysis [[10], [11], [12]]. The physicochemical properties of the modified starch are then related to the degree of substitution (DS) of the chemical groups formed and so the properties of the material can be controlled by DS [1].

The purpose of this study was to prepare thermoplastic non-acetylated and thermoplastic acetylated starch materials obtained from the same type of natural starch and characterize their various properties and especially their biodegradability. In order to verify the hypothesis that the degree of morphological homogeneity, as well as the DS, allow controlling the biodegradability of starch materials, this work also comprises the preparation of starch materials by various procedures with an emphasis on the final DS and particularly their morphological characterization. The acetylated starches with different degree of substitution were successfully synthesized and plasticized to obtain thermoplastic materials. Unfortunately, there is no universal plasticizer for these materials and proper plasticizing system has to be chosen reflecting the degree of acetylation. Materials with low degree of substitution have been plasticized by glycerol/triethylcitrate mixture. However, this mixture is not miscible with highly acetylated starch, for which neat triethylcitrate or triacetin has been used. In order to enhance the processability of the thermoplastic starch materials, systems containing maltodextrin, i.e. an oligomeric compound obtained by enzymatic treatment of starch, have been prepared and characterized as well.