Modification of Eucalyptus and Spruce organosolv lignins with fatty acids to use as filler in PLA

https://doi.org/10.1016/j.reactfunctpolym.2016.05.002Get rights and content

Abstract

Spruce (softwood) and Eucalyptus (hardwood) woods were used as raw material for lignin extraction by organosolv process. Chemical, structural and thermal characterizations of the extracted lignin samples have been performed using HPLC, GPC, FT-IR, 31P NMR, 13C NMR, DSC and TGA. Both lignins showed high purity, being Spruce lignin (OS) which presented the highest Klason lignin content (93%) and lowest sugar (0.5%) and sulphur (0.04%) content. Extracted lignin samples were chemically modified with dodecanoyl chloride fatty acid, in order to modify its thermal properties as glass transition temperature (Tg). The noticeable increase in the molecular weight and sharp decrease of Tg can be appreciated. Esterified lignins were used as filler in poly(lactic acid) (PLA) films elaborated by solvent casting in different concentrations (1, 5, 10, 25 and 50%). Mechanical, thermal and water barrier properties of prepared films were investigated. The results showed that the addition of both modified lignins contributed to greater ductility and lower stiffness, providing plasticity to PLA.

Introduction

Nowadays most of the plastics materials are made from petroleum. Although the development of polymeric materials has a vital importance in the society over the years and has contributed to facilitate our way of life, their use creates many potential problems due to their non-renewable nature and ultimate disposal [1]. Therefore, is necessary to develop biodegradable materials based on renewable sources with comparable properties to synthetic polymers and at equivalent cost. For this purpose, the use of lignocellulosic biomass is an attractive alternative due to their renewable origin, the biodegradability of their components and their non-human food application [2], [3].

Lignin, one of the main structural components of lignocellulosic biomass and the second most abundant macromolecule in nature, can offer a large amount of organic material that could be used in the production of biopolymers. The conversion of lignin to value-added products like biomaterials is an essential part of the integrated biorefinery concept. Although native lignin structure has been studied for years, its chemical structure has notable distinction depending on the origin, growing conditions of the plant [4], [5] and also employed isolation process. In general, lignin is a complex amorphous and heterogeneous polyphenol material with numerous functional groups such as methoxyl groups (–OCH3), phenolic and aliphatic hydroxyl groups principally (–OH). Its abundance and biodegradability as well as the presence of appointed functional groups that could be easily modified by well-known methods, makes it a good alternative as a raw material in the polymer industry.

Several authors showed the possibility to incorporate lignin directly without any modification into other commercial synthetic [9], [10], [11], [12], [13] and biodegradable [14], [15] polymers to give new or improved properties to the material. However, the poor compatibility between lignin and blended polymers was often observed [16], [17], Therefore, the chemical modification can be a good alternative for the functionalization of hydroxyl groups [6]. Due to its structural features, lignin has a high potential for chemical modifications, which can lead to value-added polymeric materials with specific properties [18]. Several studies have been published about esterification of lignin in an attempt to improve the dispersion in the blends and performance in the final materials [19], [20], [21]. Hence, chemical modification of functional groups presents in lignin molecule like phenolic hydroxyl groups and aliphatic hydroxyl groups at the C-α and C-γ positions on the side chain, is a good alternative to increase its range of applications in the polymer industry area [6], [22], [23]. The esterification of lignin allows increasing the hydrophobicity of lignin and its solubility in organic solvents [24]. Moreover, lignin also possesses many disadvantages like its brittleness, poor film forming ability and difficulty of processing [25]. The natural condensed structure and strong intermolecular hydrogen bonding interactions in lignin restrict the thermal mobility and this provides high Tg value [17]. When lignin is modified by esterification, hydroxyl groups are replaced by ester substituent [26] and thus, reduce the number of hydrogen bonding and lead an increased free volume in the molecule, providing mobility of the chains [27]. In this sense, esterification is a potential route to lower the glass transition point of lignin and increase its thermoplasticity [25], [28], [29], highly desirable for current industrial polymeric processing technology.

In the present study, two types of raw materials (Spruce and Eucalyptus) were used for the lignin isolation by organosolv process. First, the determination of chemical composition, structure and thermal properties of extracted lignins were analysed. Here, the determination of functional groups of the lignins, like aliphatic and phenolic hydroxyl groups are an important factor, due to provide the total content of potential active sites in the lignin polymer structures able to be modified. The knowledge of the lignin chemical structure is very important for its chemical modification and therefore for new materials elaboration.

Moreover, esterification reaction of obtained lignins was carried out using fatty acid as reagent with a long aliphatic chain (12C). This modification is highlighted for its short period without using temperature. This strategy was developed with two different objectives. First, to analyse the changes in the thermal properties of lignins, as glass transition temperature and thermal stability. And the second one, to try to improve the low ductility of the poly(lactic acid) (PLA) using modified lignins as filler in different concentrations to overcome the major drawbacks of PLA for some applications (brittleness and stiffness). In fact, PLA is a brittle and rigid polymer, which has very low ability to plastic deformation (~ 3%). The brittleness is one of the important weaknesses of the PLA [30] due to it presents low deformation at break and high modulus that limits its applications in packaging industry [31]. Besides that, in this work, the effect of esterified lignin incorporation in the PLA thermal stability, mechanical properties and water barrier properties has been analysed. Although some works have been published about lignin application into some polymeric materials systems, the incorporation of esterified lignin by fatty acids as well as its application as filler into PLA for the improvement of its material properties and the elaboration of completely renewable material result highly interesting.

Section snippets

Materials

Two different raw materials were used for lignin extraction; Spruce and Eucalyptus wood, provided by KTH University in Sweden. PLA (Nature-Works® PLA Polymer 3051D) was from NatureWorks LLC (glass transition temperature 55–65 °C, melt temperature 150–165 °C, Mw: 169.000 g/mol). Ethanol absolute (99.9%), N,N′-dimethylformamide (99%), pyridine (99%), dodecanoyl chloride (98%) and hydrochloric acid (37%) were used as supplied by Panreac and Sigma-Aldrich.

Lignin isolation

Both raw materials were subjected to

Chemical composition of isolated lignins

The chemical composition of isolated lignins was determined in terms of Klason lignin, acid soluble lignin (ASL), ashes, and sugars content. Elemental analysis was used to determine sulphur content. The impurities of lignins are usually composed by residual carbohydrates and ashes. As can be observed in Table 1, OS presented the highest lignin and less sugar content, however, the ash content is similar for both lignin samples. The high sulphur content for OS (0.20) can be due to the sulphuric

Conclusions

Chemical composition, structure and thermal properties of organosolv lignins extracted from Spruce and Eucalyptus were studied. Both lignins were esterified by fatty acid with a long chain (12C) to modify their thermal properties and to use as filler in PLA matrix to improve material properties such as the brittleness. The success of the modification reaction of lignins was confirmed by FTIR and GPC. The modification involves a significant decrease in glass transition temperature in both

Acknowledgements

The authors are grateful for the financial support received from the University of the Basque Country (doctoral grant of Ms. Gordobil Grant No. PIF 13/050 and post-doctoral training programs No. ESPDOC 14/3) and from the Spanish Ministry of Economy and Competitiveness (CTQ2013-41246-R).

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