Elsevier

Carbohydrate Polymers

Volume 92, Issue 1, 30 January 2013, Pages 810-816
Carbohydrate Polymers

Preparation and characterization of poly(lactic acid)/starch composites toughened with epoxidized soybean oil

https://doi.org/10.1016/j.carbpol.2012.09.007Get rights and content

Abstract

Blends of entirely bio-sourced polymers, namely polylactide (PLA) and starch, have been melt-compounded by lab-scale co-extruder with epoxidized soybean oil (ESO) as a reactive compatibilizer. The starch granules were grafted with the maleic anhydride (MA) to enhance its reactivity with ESO. The ready reactions between the epoxy groups on ESO, the MA groups on MA-grafted starch (MGST) and the end carboxylic acid groups of PLA brought blending components together and formed a compatible compound. An elongation at break (EB) of 140% was obtained in the blend of PLA/MGST/ESO (80/10/10), increased from 5% of a pure PLA. The grafting content of the MA on the starch granules primarily determined the compatibility and properties of the ternary blends, which was also affected by the relative amount of MGST and ESO.

Highlights

► It has no report about PLA/starch blends toughened and compatibilized by ESO. ► The elongation at break increased very significantly because of the addition of ESO. ► The miscibility and mechanical properties of PLA/starch/ESO ternary blends could be further improved via the MA grafting native starch. ► ESO droplets were not observed in PLA matrix after MA grafting starch in Fig. 3.

Introduction

With the environmental concerns and the scarce supply of the crude oil, a revival in the development of bio-based and compostable materials is coming. They are in general being called the “green” materials (Mohanty et al., 2002, Mohanty et al., 2000), among which, poly(lactic acid) (PLA) got the most extensive attention due to its good combined properties, including the renewable feedstock, the compostability, and its good mechanical properties, especially the Young's modulus and the stiffness.

Currently, PLA has found numerous applications in, for example, the bi-axially oriented films, the thermoformed food/beverage serving utensils, and the injection stretch blow molding (ISBM) bottles (Tweed, Stephens, & Riegert, 2006). Thus, among all the synthetic biopolymers, PLA is considered having the highest potential to replace some regular petroleum based plastics. However, PLA has its own shortcomings, i.e., its brittleness, low heat distortion temperature (HDT) (Tang et al., 2012), and relatively high cost, which limit its broad acceptance (Anderson et al., 2008, Auras et al., 2004). Plasticizers were used to improve PLA's impact strength and low cost fillers to bring down its total cost (Jacobsen et al., 2000, Huda et al., 2005).

As a low cost nature renewable material, starch is a good filler candidate for the PLA. However, they are not chemically compatible (Wang, Sun, & Seib, 2001), and in addition, starch granules might make the PLA even more brittle. To solve these issues, two general measures were taken: (a) using compatibilizers between PLA and the starch granules; and (b) plasticizing the starch granules to make them easier dispersed into PLA matrix.

Some examples of the measure (a) including maleic anhydride (MA) (Zhang & Sun, 2004), acrylic acid (AA) (Wu, 2005) and methylene-diphenyldiisocyanate (MDI) (Wang et al., 2001),etc. were used as the reactive compatibilizers. However, all above referred coupling agents used in PLA/starch blends are inefficient on the improvement of toughness.

As a general way of the measure (b), starch plasticizers, such as water (Teixeira et al., 2007), glycerol (Rodriguez-Gonzalez, Ramsay, & Favis, 2004), sorbitol (Li & Huneault, 2011), urea (Ma, Yu, & Wan, 2006) and citric acid (Shi et al., 2007), were used to break the crystallinity and the strong hydrogen bond within the starch granules. Under the melt blending process, these small molecular plasticizers destroy the starch granules and make the plasticized starch, in essence, a thermoplastic starch (TPS), more compatible with PLA matrix. A problem of the above small molecular plasticizers is that they are hydrophilic chemicals and could accelerate the hydrolysis degradation of PLA in the melt-blended process. Moreover, these small molecules may migrate out of the blends and deteriorate the blending system as well as cause the retrogradation of the starch.

To prevent the migration of the small plasticizers, chemical bond can be generated between the plasticizers and the starch. Wang, Zhai, and Zheng (2012) caped polyethylene glycol (PEG), a plasticizer, with MA and then grafted this MA-PEG-MA onto the native starch. This modified starch was then blended with PLA and more PEG plasticizer from an extruder. The resultant compound showed good compatibility between MA-PEG-MA modified starch and the PLA. Free PEG was trapped within the interface of MA-PEG-MA modified starch and PLA, well plasticizing the starch and improving the compatibility between the starch and PLA, turning PLA matrix from a brittle state into a ductile state. However, the grafting content of MA-PEG-MA onto starch was low due to the slow esterification reaction between MA and the starch, therefore the above enhanced compatibility effect was also limited. Meanwhile, the blending plasticizer PEG used was derived from petroleum.

Recently, bio-based plasticizers have attracted the general attention due to their renewability. Epoxidized soybean oil (ESO) is one of such plasticizers. It is manufactured through the process of epoxidation of the carbon–carbon double bonds on the aliphatic long chain moiety of the soybean oil molecules. ESO is mainly used as a plasticizer for PVC and chlorinated rubber (Ishiaku et al., 1997). Ali et al. (2009) used ESO as an effective plasticizer for PLA, which depressed PLA's glass transition temperature and improved its cold crystallization capability. Broström, Boss, and Chronakis (2004) studied the effects of ESO on the morphology and the mechanical properties of PLLA-co-PCL/triethyl citrate blends. It was shown that the compatibility between triethyl citrate and the PLLA-co-PCL copolymer was improved in the presence of ESO with the EB of the blend increased significantly to 272%. It was proposed that the reactions between epoxy groups on ESO and hydroxyl and/or carboxyl groups on PLLA-co-PCL, as well as the hydroxyl group on triethyl citrate (as a plasticizer) generated the chemical interaction within the system and also improved the toughness of the compound.

Considering that there were abundant hydroxyl groups provided by the starch molecules and ester carboxyl groups existed on PLA. Thus, ESO might also be used as a reactive plasticizer in PLA/starch system. Very few studies have been made in this area.

Section snippets

Materials

The PLA 4032D, a semi-crystalline extrusion grade, was supplied by NatureWorks LLC (Minnesota, USA). It was vacuum dried at 80 °C for at least 8 h before use. The food grade native corn-starch was obtained from the Zhucheng Stimulation Trade and Corn Development Limited Company, Shangdong, China, which was dried in vacuum for 24 h at 100 °C before use. The chemical pure grade ESO was purchased from the Aladdin Reagent (Shanghai, Chain) and used directly without further purification. The analytical

The FTIR analysis of MGST with various DS

Fig. 1 shows the FTIR spectra of MGST with various DS. The peak at 1731 cm−1 represents the Cdouble bondO stretching vibration peak from MA grafted starch, which does not show for the native starch. The intensity of this peak increases with the increase of DS, which reflects the amount of the MA groups grafted.

Thermal properties

The pure PLA shows a glass transition at 61 °C (Tg), an exothermic cold crystallization peak at 117 °C (Tc) and a sharp endotherm peak at 164 °C with a shoulder peak to its right. (Fig. 2a). This

Conclusions

Epoxidized soy-bean oil (ESO) could be used as a bio-based reactive plasticizer for PLA and the starch compounds. This effect could be enhanced by chemically grafting the starch granules with the maleic anhydride (MA). The characterization of the obtained blends by differential scanning calorimetry (DSC) demonstrated that the cold crystallization temperature (Tc) of PLA was obviously declined by the addition of ESO, whereas the content of 5 wt.% ESO in PLA/MGST blends was the key point. The

Acknowledgements

The authors gratefully acknowledge Dr. Jun Wang (Colgate-Palmolive researcher) for article reviewing. We also thank Dr. Xiaoqin Liu for his valuable help.

References (28)

  • R. Auras et al.

    An overview of polylactides as packaging materials

    Macromolecular Bioscience

    (2004)
  • J. Broström et al.

    Biodegradable films of partly branched poly(l-lactide)-co-poly(epsilon-caprolactone) copolymer: Modulation of phase morphology, plasticization properties and thermal depolymerization

    Biomacromolecules

    (2004)
  • K. Chang et al.

    Phase inversion in polylactide/soybean oil blends compatibilized by poly(isoprene-b-lactide) block copolymers

    Applied Materials and Interfaces

    (2009)
  • W.M. Gramlich et al.

    Reactive compatibilization of poly(l-lactide) and conjugated soybean oil

    Macromolecules

    (2010)
  • Cited by (194)

    View all citing articles on Scopus
    View full text