Thermoplasticization of euglenoid β-1,3-glucans by mixed esterification

Highlights

• Thermoplastics made from paramylon as a main component were prepared.

• Euglenoid constitution ratio of plastics theoretically rises to as high as 70%.

• Creation of thermoplastics hinges on mixed esterification.

• Thermoplastics are of practical use in terms of thermal and mechanical properties.

Abstract

We experimentally demonstrated that paramylon, a storage polysaccharide of Euglena gracilis, is efficiently thermoplasticized by adding acyl groups that differ in alkyl chain length. Glass transition temperature of mixed paramylon esters was higher than those of plant-based polylactic acid (PLA), poly 11-aminoundecanoic acid (PA11), and petroleum-based acrylonitrile-butadiene-styrene (ABS) resin and was comparable to that of cellulose acetate stearate (CAS). Their thermoplasticity was equivalent to or higher than those of these reference plastics. The bending strength and bending elastic modulus of injection molded test specimens made from mixed paramylon esters were comparable to those of the reference plastics. While their impact strength was lower than that of specimens made from ABS resin and CAS, it was comparable to those of PLA and PA11. Euglenoid β-1,3-glucans are thus a potential component of thermoplastic materials.

Introduction

The use of microalgae to produce useful chemicals has recently attracted attention because they can transform carbon dioxide and water into carbohydrates by photosynthesis (Chisti, 2007). Attention has particularly focused on production of biofuels as part of the effort to reduce dependence on carbon-based fuels (Banerjee et al., 2002, Kaya et al., 2011, Mata et al., 2010, Tucci et al., 2010, Yamane et al., 2013). By contrast, less attention has been directed at the use of microalgae to create other useful difficult-to-synthesize carbohydrates. Our efforts in this area have focused on Euglena-derived storage polysaccharide (β-1,3-glucan), which is generally referred to as paramylon (Shibakami et al., 2012, Shibakami et al., 2013).

Since paramylon has unique helical structures due to its β-1,3-bonds, it is likely that materials made from this polysaccharide will exhibit intriguing thermal and mechanical properties that differ from those of materials made from other polysaccharides. One physical property that is useful for preparing materials of various shapes is thermoplasticity. Paramylon, however, does not inherently have this property. It has been reported that mixed cellulose esters having at least two types of substituents exhibit thermoplasticity (Peydecastaing et al., 2011, Teramoto et al., 2002). Wax esters, i.e., esters of long-chain fatty acid and higher alcohol, are another major euglenoid product in addition to paramylon. Of particular interest is that hydrolysis of wax esters releases the fatty acid and higher alcohol. We hypothesized that if wax ester-derived fatty acids are introduced into paramylon, we will likely obtain thermoplastic mixed paramylon esters with a high euglenoid constituent ratio. Toward this goal, we have started a program aimed at synthesizing mixed paramylon esters containing long-chain fatty acids.

Here we report the creation of thermoplastic paramylon derivatives. To the best of our knowledge, this is the first creation of Euglena-based thermoplastics despite the long study of Euglena since the establishment of genus Euglena by Ehrenberg in 1830 (Gojdics, 1953). The primary objective of the work described here was twofold: (i) to establish a mix-esterification method that provides thermoplastic paramylon derivatives and (ii) to investigate the relationship between the chemical structures of the derivatives and their thermal and mechanical properties.