Effect of biodegradable plasticizers on thermal and mechanical properties of poly(3-hydroxybutyrate)

Polymer Testing 23 (2004) 455–460 www.elsevier.com/locate/polytest

Material Properties

Effect of biodegradable plasticizers on thermal and mechanical properties of poly(3-hydroxybutyrate) Jae Shin Choi, Won Ho Park ∗ Department of Textile Engineering, College of Engineering, Chungnam National University, Daejeon 305-764, South Korea Received 22 July 2003; accepted 4 September 2003

Abstract The effects of biodegradable plasticizers on the thermal and mechanical properties of poly (3-hydroxybutyrate-co3-hydroxyvalerate) (PHBV) were studied using thermal and mechanical analyses. Soybean oil (SO), epoxidized soybean oil (ESO), dibutyl phthalate (DBP) and triethyl citrate (TEC) were used as plasticizing additives. PHBV/plasticizer blends were prepared by evaporating solvent from blend solutions. The content of plasticizer in the blends was kept at 20wt%. Compatibility of plasticizer with PHBV was examined with differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). DPB and TEC were more effective than soybean oils (SO and ESO) in depression of the glass transition temperatures as well as in increasing the elongation at break and the impact strength of the films. From the thermal and mechanical properties of the plasticized PHBV, it could be concluded that TEC or DBP are better plasticizers than SO and ESO for PHBV.  2003 Elsevier Ltd. All rights reserved. Keywords: Poly(3-hydroxybutyrate-co-3-hydroxyvalerate); Plasticizer; Blend; Mechanical properties

1. Introduction Poly(3-hydroxybutyrate) (PHB), which is biosynthesized by a wide variety of bacteria, is a biodegradable thermoplastic with biocompatibility and ecological safety. Therefore, it has great potential in applications such as surgical sutures, long-term carriers for drugs, and degradable plastics. However, currently there are not many commercial products because PHB has higher cost and narrow processability window and brittleness, compared to commercial synthetic polymers. These drawbacks in PHB can be overcome by internal or external plasticization to improve impact strength, elongation at break and ductility. From many attempts for the internal plasticization of PHB, various copolymers with 3-hydroxyvalerate [1–3], 4-hydroxybutyrate [4–6], and 3-hydroxyalkanote units [7–9] have been developed using the

∗

Corresponding author. Fax: +82-42-823-3736. E-mail address: [email protected] (W.H. Park).

bacterial fermentation method. These copolymers have improved physical and mechanical properties, compared to PHB homopolymer. On the other hand, the external plasticization is more efficient to apply to polymers because it could provide a relatively simple route to improve some mechanical properties of polymer as well as to lower costs. However, plasticizers for biodegradable polymers should preferably also be biodegradable. In this respect, most of the plasticizers used in synthetic polymer processing are not suitable for PHB. There have been only a few reports [10–13] on the external plasticization of PHB, though biodegradable plasticizers play an important role in the practical processing for some applications such as packaging materials. The objective of this study is to develop the appropriate plasticizer for improving the stiffness and thermal stability of PHB. We focused on the comparison of physical and mechanical properties of PHB blends, plasticized by biodegradable additives at constant level of 20 wt%.

0142-9418/$ - see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2003.09.005

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2. Experimental

2.2. Preparation of PHB/plasticizer blends

2.1. Materials

PHBV/plasticizer blends (80/20, w/w) were prepared by evaporating chloroform from polymer/plasticizer mixed solution (3 wt%) with pre-determined weight ratios. The resulting precipitates were dried in vacuo at 30 °C. The blend films were obtained using a hot press (Alpha Engineering Co.) at 190 °C for 45 s. The thickness of the films was about 100 µm. The codes of the blend samples are listed in Table 2.

The PHBV sample used in the present work was kindly supplied by Zeneca Co as an injection-molding grade in chip form. The PHBV copolymer consisted of 3-hydroxybutyrate units 94% and 3-hydroxyvalerate units 6%, as determined by 1H NMR spectrum from a Bruker AC-200 NMR spectrometer. Soybean oil (SO), dibutyl phthalate (DBP) and triethyl citrate (TEC) were purchased from Aldrich Co. Epoxidized soybean oil (ESO) with epoxide content of 6.9 wt% was supplied by Shindongbang Co. Epoxide content (6.9 wt%) of ESO indicates that approximately 95% of unsaturated groups in SO was epoxidized. The chemical structure, molecular weight, and melting temperature of polymer and plasticizers are shown in Table 1.

2.3. Measurements 1

H NMR spectra were obtained with a Bruker AC200 NMR spectrometer. Chloroform-d solutions of the polymer at concentration of 10 mg/ml were analyzed. Differential scanning calorimetry (DSC) was conducted using a TA Instruments 2920 (duPont Co.), calibrated

Table 1 Chemical structure, molecular weight and melting temperature of polymer and plasticizers Chemical structure

Molecular weight

Tm (°C)

PHBV

680,000

161.0

SO

814.3

⫺30.4

ESO

872.2

⫺15.7

DBP

278.2

⫺35.0

TEC

276.1

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Table 2 Codes of PHBV/plasticizers

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Table 3 DSC results of PHBV/plasticizer blends

Blend investigated. PHBV/plasticizer (wt ratio:80/20)

Code

PHBV/no plasticizer PHBV/soy bean oil PHBV/epoxidized soybean oil PHBV/dibutyl phthalate PHBV/triethyl citrate

PHBV SO 20 ESO 20 DBP 20 TEC 20

with standards. Samples were heated from 20 to 190 °C at a rate of 10 °C/min (first heating) and held at final temperature for 1 min to eliminate the thermal history applied to the samples. After cooling to ⫺100 °C, they were then reheated to 200 °C at a rate of 10 °C/min (second heating). Thermal parameters such as glass transition temperature (Tg), melting temperature (Tm), and cold-crystallization temperature (Tc) were obtained from the second run. Thermogravimetric analysis (TGA) was conducted using a TA Instruments 2910 (duPont Co.) in a nitrogen atmosphere from 30 to 500 °C at a heating rate of 20 °C/min. Tensile properties of the dumbbell-shaped PHBV blend samples were carried out to ASTM D638-5 on an Instron (Model 1137) at a crosshead speed of 5 mm/min. Impact strength of the notched samples was obtained on a Toyoseiki Model DG-1B according to ASTM D256. The samples for impact test were injection-molded at 190 °C by a Mini-Max molder using the solutionblended samples. Cyrogenic fracture surfaces of the blend films were examined by scanning electron microscopy (SEM) (ISISX-40, Akashi Co.) with an accelerating voltage of 15 kV. The samples for SEM observation were selectively etched in methanol for 1 h at room temperature.

3. Results and discussion 3.1. Thermal properties of PHBV/plasticizer blends To investigate the efficiency of four plasticizers (SO, ESO, DBP and TEC) on PHBV, DSC was carried out using the PHBV blends with constant plasticizer content of 20wt%. The thermal properties of Tg, Tc, Tm and ⌬Hm of PHB/plasticizer blends, obtained from a second heating, are summarized in Table 3. A depression of glass transition temperatures was observed in PHBV/plasticizer blends, but not in PHB/SO blends. Effective plasticization induces the depression of the glass transition temperature in polymer/plasticizer system. Therefore, ESO-20, DBP-20 and TEC-20 were expected to be compatible with PHBV and improve the

Sample code

Tg(°C)

Tc(°C)

Tm(°C)

⌬Hm(J/g)

PHBV SO 20 ESO 20 DBP 20 TEC 20

⫺6.6 ⫺3.4 ⫺19.0 ⫺28.5 ⫺30.0

28.5 41.7 39.0 17.3 22.9

161.0 161.6 159.5 152.7 144.4

65.9 64.5 61.6 59.1 53.0

mobility of PHB molecular chains in the amorphous phase. The Tg of TEC-20 shifted to the lowest temperature and DBP-20 followed in second place. In addition, the Tm value and normalized heat of fusion (⌬Hm) of TEC-20 and DBP-20 showed lower values than pure PHBV, while they did not change in SO-20 and ESO-20. In general, the melting temperature reflects the lamella thickness. This result suggests that TEC-20 and DBP-20 have thinner lamella. The cold crystallization temperature (Tc) of TEC-20 and DBP-20 showed lower values than pure PHBV, while it increased in SO-20 and ESO20. From the thermal properties of PHBV blends, it can be concluded that the efficiency of plasticizer on PHBV was higher in the order of TEC - 20 ⬎ DBP - 20 ⬎ ESO - 20 ⬎ SO - 20. 3.2. Mechanical properties of PHBV/plasticizer blends In order to ascertain if the brittleness of PHBV could be improved by blending with plasticizer, the tensile properties of PHBV/plasticizer blends were compared with those of pure PHBV. Strength, modulus and elongation at break of PHBV and its blends are shown in Fig. 1. The elongation at break as well as strength and modulus of SO-20 decreased. However, the elongation at break of ESO-20, DBP-20 and TEC-20 were considerably improved, whereas their strength and modulus showed lower values than pure PHBV. This could be also verified from the notched Izod impact test results of PHBV blends. Fig. 2 shows the impact strength of PHBV blend films with different plasticizer. The impact strength values of PHBV blends were more than three times that of PHBV except SO-20. The results from the elongation at break and impact strength indicated that the flexibility of blends increased in the order of TEC - 20⬇DBP - 20 ⬎ ESO - 20 ⬎ SO - 20. Fig. 3 shows SEM micrographs of the etched fracture surfaces of PHBV blends. As seen in figures, the domain size in TEC-20 and DBP-20 were smaller and more uniform than in SO-20 and ESO-20. This observation suggests that SO and ESO were less compatible with PHBV than TEC and DBP, and accordingly the segregation of SO or ESO in the blends occurred. Therefore, the

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Fig. 1.

Tensile properties of PHBV/plasticizer blends.

Fig. 2. Impact strength of PHBV/plasticizer blends.

improved ductility in TEC-20 and DBP-20 could be attributed to their compatibility with PHBV. 3.3. Efficiency of plasticizers on PHBV The efficiency of a plasticizer is dependent on its chemical structure, molecular weight and concentration. Plasticization requires that the plasticizer interacts with the polymer chains distributing itself uniformly within the polymer, hence creating additional free volume. Compatiblity or solubility of the plasticizer with the polymer is, therefore, of major significance for effective plasticization. In general, the effectiveness and compatibility of the plasticizer typically requires a similarity in solubility parameters, in particular polarity and hydrogen bonding between polymer and plasticizer [14]. Therefore, the effect of plasticizers on thermal and mechanica l properties of PHBV can be explained in terms of solubility parameters of the polymer and plasticizer. The solubility parameters of PHBV and plasticizers were cal-

culated from the Hansen parameters [15], which are divided into three-component solubility parameters to account for the dispersion, the polar interactions and hydrogen bonding. The calculated solubility parameters of PHBV and plasticizer are listed in Table 4. The solubility parameter (d), and its polar and hydrogen bonding components (dp and dh) of DBP or TEC is closer to the corresponding values of PH BV than those of SO or ESO, whereas the dispersion component (dd) of solubility parameter of SO and ESO is closer to the corresponding values of PHBV than that of DBP or TEC. This indicates that a similarity in the polar and/or hydrogen bonding components of solubility parameter between polymer and plasticizer could be an important factor for the effectiveness and compatibility of the plasticizer. 4. Conclusions The ultimate aim of this study is to develop the appropriate plasticizer for improving the mechanical proper-

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Fig. 3.

Scanning electron micrographs of PHBV/plasticizer blends.

Table 4 Solubility parameters of PHBV and plasticizers

PHBV SO ESO DBP TEC

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Acknowledgements

d(MPa1/2)

dd((MPa1/2)

dp(MPa1/2)

dh(MPa1/2)

This work was supported by Korea Research Foundation Grant (KRF-2000-041-E00432).

20.6 16.2 16.8 19.1 23.8

16.5 15.5 16.5 17.9 20.3

8.8 1.4 1.6 4.1 3.6

8.6 4.7 5.1 5.3 11.8

References

ties, particularly stiffness, of poly(3-hydroxybutyrate-co3-hydroxyvalerate), PHBV. For this, soybean oil (SO), epoxidized soybean oil (E SO), dibutyl phthalate (DBP) and triethyl citrate (TEC) were used as biodegradable plasticizer for PHBV. From the thermal properties including glass transition temperature of blends, the efficiency of plasticizer was higher in the order of TEC ⬎ DBP ⬎ ESO ⬎ SO. The elongation at break and impact strength of PHBV/TEC and PHBV/DBP blends were also higher than PHBV/SO and PHBV/ESO blends. The effective plasticization of TEC and DBP could be confirmed by SEM observation and solubility parameter consideration.

[1] P.A. Holmes, Phys. Technol. 16 (1985) 32–36. [2] Y. Doi, M. Kunioka, Y. Nakamura, K. Soga, Macromolecules 20 (1987) 2988–2991. [3] Y. Doi, A. Tamaki, M. Kunioka, K. Soga, Appl. Microbiol. Biotechnol. 28 (1988) 330–334. [4] M. Kunioka, Y. Nakamura, Y. Doi, Polym. Commun. 29 (1988) 174–176. [5] Y. Doi, M. Kunioka, Y. Nakamura, K. Soga, Macromolecules 21 (1988) 2722–2727. [6] M. Kunioka, Y. Kawaguchi, Y. Doi, Appl. Microbiol. Biotechnol. 35 (1989) 569–573. [7] H. Brandl, R.A. Gross, R.W. Lenz, R.C. Fuller, Appl. Environ. Microbiol. 54 (1988) 1977–1982. [8] R.A. Gross, C. DeMollo, R.W. Lenz, R.C. Fuller, Macromolecules 22 (1989) 1106–1115. [9] R.G. Lageveen, G.W. Huisman, H. Preusting, P. Ketelaar, G. Eggink, B. Witholt, Appl. Environ. Microbiol. 54 (1988) 2924–2932.

460

J.S. Choi, W.H. Park / Polymer Testing 23 (2004) 455–460

[10] K. Ishikawa, Y. Kawaguchi, Y. Doi, Kobunshi Ronbunshu 48 (1991) 221–226. [11] G. Cecorulli, M. Pizzoli, M. Scandola, Macromolecules 25 (1992) 3304–3306. [12] N. Yoshie, K. Nakasato, M. Fujiwara, K. Kasuya, H. Abe, Y. Doi, Y. Inoue, Polymer 41 (2000) 3227–3234.

[13] L. Savenkova, Z. Gercberga, V. Nikolaeva, A. Dzene, I. Bibers, M. Kalnin, Process Biochem. 35 (2000) 573–579. [14] L.V. Labrecque, V. Dave, R.A. Gross, S.P. McCarthy, ANTEC ‘95 (1995) 1819–1823. [15] C.M. Hansen, K. Skaarup, J. Paint Technol. 39 (1967) 511–517.