Natural additives for poly (hydroxybutyrate - CO - hydroxyvalerate) - PHBV: effect on mechanical properties and biodegradation

Natural additives for poly (hydroxybutyrate - CO - hydroxyvalerate) - PHBV: effect on mechanical properties and biodegradation

Daiane Gomes Brunel^I; Wagner Maurício PachekoskiII, ^*; Carla Dalmolin^III; José Augusto Marcondes Agnelli^I

^IDepartamento de Engenharia de Materiais, Universidade Federal de São Carlos - UFSCar, Rod. Washington Luis, Km 235, CEP 13565-905, São Carlos, SP, Brasil

^IIUniversidade Federal de Santa Catarina - UFSC, Campus Joinville, Rua Presidente Prudente de Moraes, 406, CEP 89218-000, Joinville, SC, Brasil

^IIIDepartamento de Química, Centro de Ciências Tecnológicas - CCT, Universidade do Estado de Santa Catarina - UDESC, Rua Paulo Malschitzki, s/n, Campus Universitário Prof. Avelino Marcante, CEP 89219-710, Joinville, SC, Brasil

ABSTRACT

In this work, the improvement of mechanical properties in biodegradable materials was obtained through the incorporation of natural and also biodegradable plasticizers and nucleation agents into the PHBV copolymer. PHBV production with different quantities of additives was obtained by extrusion followed by injection. The additives in the copolymer were efficient, resulting in an adequate processing due to the presence of nucleate and an improvement of the mechanical properties of the resulting material provided by the action of the plasticizer. The formulation with the minimum amount of additive content, 5% epoxidized cottonseed oil and 0.1% Licowax, was the most effective showing 35% reduction in the elastic modulus, and 18% in the PHBV crystallinity; 58% increase in impact resistance and 46% increase in elongation. Furthermore, it is important to emphasize that the natural additives were very efficient for biodegradation, showing a mass loss higher of pure PHBV.

Keywords: poly(hydroxybutyrate-co-hydroxyvalerate), PHBV, biodegradable polymers, additives, mechanical properties

1. INTRODUCTION

Poly (3-hydroxybutyrate) - PHB - is a well-known biologically derived and biodegradable polymer¹. Since it can be produced from renewable resources, it has received increasing attention due to the potential applications such as in environment-friendly products, tissue engineering, and control release devices^2,3. Nowadays, bacterial fermentation is the main source for PHB production. The process basically consists of two stages: a fermentative stage, in which the microorganisms are fed in reactors containing butyric acid or fructose, where they metabolize the sugar available and accumulate the PHB in the inner cell as a power supply source; and the extraction stage, where the polymer accumulated in the microorganism inner cell is removed and purified with adequate solvents until obtaining the final product, that is solid and dry⁴. However, the commercialization of these materials did not result in a major replacement of the conventional plastics because of the higher costs of PHB, its brittleness, and a narrow process window due to the lack of thermal stability. The PHB copolymer, i.e. poly (3-hydroxybutyrate-co-3-hydroxyvalerate) - PHBV - has been developed in an effort to improve its properties for industrial application. It is produced by a fermentative process similar to the PHB process, only differing in the use of propionic acid, together with glucose, as a carbon source. The amount of propionic acid that is found in the nourishment of the bacteria is responsible for the concentration of hydroxyvalerate (HV) in the copolymer. As HV content increases, T_G, T_M and crystallinity decrease, improving the processing and toughness in PHB⁵.

By being thermoplastic, of renewable sources, biodegradable, compostable and biocompatible, PHB and PHBV are of great interest in the production of fast usage products, such as disposable materials, packages, medical artifacts for human or veterinary, automobile industry products, among others. To be suitable for these industrial applications, however, PHB and PHBV should be processed in large scale, mostly by melt processing techniques such as extrusion and/or injection. In this case, the polymeric chains are submitted not only to high temperatures, but also to shearing tension, which may lead to a scission on the polymeric chain, causing reduction in the molar mass and characterizing a further degradation⁶. Incorporation of additives is another resource to modify some polymer properties in order to achieve better processing or to adjust their mechanical and thermal behavior. However, when dealing with biodegradable polymers, it is preferable that these additives are biodegradable as well. Indeed, some authors have reported the use of soybean oil⁷, β-carotene⁸ and low molecular weight additives⁹ as plasticizing agents for PHB and PHBV in order to improve their mechanical properties for industrial applications. Other approaches also include the use of nucleating agents and compatibilizers to accelerate the crystallization process and refine morphology and thermal stabilizers, also known as antioxidants, which can prevent various effects such as oxidation, chain scission and uncontrolled recombination that may occur during the process¹⁰.

Among various natural biodegradable additives, an epoxidized cottonseed oil plasticizer and a nucleate based on fatty acids were efficient in the improvement of processing by extrusion and injection. Futhermore, the mechanical properties and biodegradation increased when they were mixed together in a PHBV formulation¹¹. However, optimized results still can be obtained through the study of the influence of both plasticizer and nucleating agent when their contents inside the formulation are changed. Therefore, different PHBV formulations with an epoxidized cottonseed oil as the plasticizer and a nucleate based on fatty acids were processed by both extrusion and injection to result in materials with adequate mechanical properties for industrial usage and composition with approximately 100% weight in biodegradable materials. To evaluate the better composition and the effect of additives in PHBV properties, specimens with different formulations were tested by mechanical, thermal, microscopy and biodegradation analysis.

2. EXPERIMENTAL

PHBV - ( of 650,000 g/mol; 3.5% HV, 1.22 g/cm³) was manufactured by biological fermentation from renewable sugarcane carbohydrate at PHB Industrial S/A. P902^® (Logos Química Ltda), an epoxidized cottonseed oil, was chosen as the plasticizing agent, and the nucleating agent was the fatty acid based compound Licowax^® (Clariant). Contents of plasticizer (P) and nucleate (N) are listed in Table 1 for all formulations in this study. To guarantee the homogeneity between both powder PHBV and liquid plasticizer, the different blends were mixed into a Henschel blender for 10 minutes, with 450 rpm rotation. These compounds were dried in an air circulation oven (Soc. Fabbe 170) at 60°C for 24h. The different amounts of nucleating agents were manually blended. The pure copolymer and the different formulations described in Table 1 were processed in a DC-R 30:40 IF Imacom co-rotational double screw thread extruder. Next, the pelletized formulations were dried in an oven at 60°C for 24h. The injection of impact and tension specimen according to ASTM D-638^[12] and D-256 standards¹³ was carried out in a 270V 300-120 Arburg All Rounder injector, with 12 cm³/s flow and 20 cm/s injection speed.

An Instron 5569 Universal Test Machine, in ASTM D-638 standard¹², with a 10 mm clutch gap, 5 mm/min speed and 50 kN load cell was used to measure mechanical properties (Young's modulus, stress and elongation at break). The notched Izod impact test was carried out in a 65451000 code CEAST Impact Machine, with a 2 J pendulum, under controlled temperature, according to ASTM D-256 standard¹³. All procedures were done in triplicate, three days after processing.

The thermo gravimetric (TG) and the derived thermo gravimetric (DTG) curves were obtained in a TA Instrument TGA2950 at a 20°C.min ^{- 1} heating rate between room temperature (23°C) and 600°C, under N₂ atmosphere (50 mL/min) in an alumina sample rest. A TA Instruments DSC Q100 calorimeter was used for the DSC characterization ranging from - 50 to 200°C at both a heating and cooling rate of 20°C.min ^{- 1}, under N₂ atmosphere. The PHBV crystallinity was calculated by dividing the heat of fusion of each sample (ΔH_M) and the heat of fusion of the hypothetically 100% crystalline PHB, determined as 146 J/g^[14].

The influence of nucleating agent contents in the spherulites growth rate in relation to temperature was studied by optical microscopy with polarized light, using a heating plate. The optical microscope used had a DMRXP Leica polarized light and a KAPPA webcam coupled to a computer with software for capturing images. To get the experiments into controlled temperatures, a THMS 600 Linkan heating plate was used, monitored by a TMS92 Linkan temperature controller. The samples were heated at 50°C.min ^{- 1} up to 190°C and were kept at this temperature for 3 minutes to guarantee the complete fusion of the spherulites, destroying the previous thermal history, but paying attention not to initiate a possible thermal degradation process. Subsequently, the samples were cooled down at 100°C.min ^{- 1}, up to the isothermal crystallization temperature (60°C, 70°C, 80°C, and 90°C), and were kept there for 20 minutes.

To evaluate the biodegradation of the processed copolymer, the Sturm methodology was used, which is considered the most trustable for the evaluation of polymer biodegradability in active microbial medium^15,16. This methodology consists of embedding the test specimen in an activated organic compound and evaluating its biodegradation through the mass loss and the modification in its visual aspect. In this study, the test was carried out in a Compound Organic Fertilizer (40% minimum organic matter, 45% maximum humidity, pH 6 and 18/1 maximum C/N ratio), supplied by PROVASO, under room temperature and controlled humidity, according to ASTM D-6003^[17] and ASTM G-160 standards¹⁸. Three distinct systems were prepared for withdrawing after 60, 120 and 180 days of test. Each sample system was formed by 5 tension specimens for each compound. Besides the mass loss after the biodegradable tests, samples were analyzed by Scanning Electron Microscopy (SEM) (Stereoscan 440), and the modification of the mechanical properties was evaluated by mechanical tests.

3. RESULTS AND DISCUSSION

During the extrusion, pure PHBV had high cast viscosity and slow extruded crystallization, characteristics that resulted in higher pelletizing difficulties. The formulations with additives were easier to processing, because the joint action between the plasticizer and the nucleate resulted in a more stable extrusion. The larger the amount of nucleating agent content in the formulation, the faster the extruded filament became rigid, showing good stability during extrusion. The temperature profile was similar for the pure copolymer and also for the additive formulations; however, less darkening in the compounds with additives (Figure 1) was observed, suggesting a lower thermal degradation. In the injection mold, where the same conditions were used in the processing of all formulations, it was observed that the higher the percentage of nucleate in the formulation, the more pressure in injection was necessary. The formulations with 0.5% nucleate, by crystallizing faster than the others, did not show homogeneity in the filling of the mold cavities, with constant failure in the injected specimen.

3.1. Mechanical tests

Figures 2a-c shows the PHBV copolymer stress-strain curves compared to the additive formulations with 0.1%, 0.3% and 0.5% nucleate contents and varied plasticizer contents. All the additive formulations showed lower stress in the rupture, lower elastic modulus and higher deformation if compared to the formulation without additives, indicating the effectiveness of the plasticizer in reducing rigidity and fragility of the copolymer, according to the values presented in Table 2. Comparing the results of additive formulations, it is observed that maintaining the concentration of the plasticizer constant, the increase in the nucleate content causes a reduction of strain in the rupture. Moreover, this effect is minimized when the nucleate and plasticizer contents increase. It is verified that the formulation with a minimum amount of nucleate and plasticizer (5P01N) showed a reduction of PHBV's stiffness together with a higher tensile strength, implying an improvement of its mechanical properties.

According to Table 2, the formulation with pure copolymer had the lowest impact resistance of all tested formulations. These results also showed that the PHBV impact resistance rose up to 60% due to the presence of additives; however, it was observed that increasing the quantity of the nucleating agent and/or the plasticizer content caused a reduction of the measured impact resistance. The same performance was observed in the stress and strain in rupture, where the most efficient formulation was that with the lower concentration of plasticizer and nucleates, 5P01N.

3.2. Thermal analysis

Pure PHBV and other additive formulations were submitted to DSC thermal analysis with the purpose of verifying the alterations in transition temperatures caused by the different tested additive contents. The DSC curves referent to the second heating run for pure PHBV compared with the curves for formulations with 0.1%, 0.3%, or 0.5% of nucleate and varied plasticizer contents are presented in Figures 3a-c. It can be observed that all formulations showed two melting temperatures. The first and lower temperature, characterized by a small peak in the DSC curve, corresponds to the melting of the crystalline poly (hydroxyvalerate), while the second temperature corresponds to the melting of the poly(hydroxybutyrate)^19,20. Lower melting temperatures were observed for the additive formulations when compared to the pure PHBV; however, the increase of the percentage of plasticizer and nucleate did not have a significant influence in this variation.

The values of the thermal properties (T_g - glass transition; T_cc - cold crystallization; T_m - melting point; and crystalline degree) obtained by DSC during the second heating run are found in Table 3. It was observed that T_g was reduced with the increase of the plasticizer content and, in general, the increase of the amount of nucleate in the formulation restricted this effect. In general, the additive formulations differed from the pure PHBV with the displacement of T_g, T_cc and T_m down to lower temperatures. In the formulations with 0.1% nucleate, it can be noted that with the increase in the plasticizer content, there was a gradual reduction of the melting, crystal and vitreous transition temperatures. In compounds with 0.3% nucleate, the increase of plasticizer from 5% to 7% significantly reduced the temperatures; however, the increase from 7% to 10% shows a similarity of curves, which means that the addition of more plasticizer did not reduce the temperature any more. This behavior suggests a possible exudation of the plasticizer. The formulations with plasticizer and 0.5% nucleate had analogous behavior.

It can be observed an important reduction in the T_cc of the additive formulations when compared with pure PHBV (from 68°C to 36-40°C). This reduction causes a delay on the melting stiffness and, consequently, increases the time needed for the cooling and molding releasing steps. This effect explains the difficulty found during the processing for the injection of the compositions with high additive contents. Also, the presence of additives in the formulation made the PHBV crystallization more difficult, reducing its crystallinity. The increase in the nucleating content did not cause an increase in crystallinity.

PHBV based materials were examined by TGA combined to its first derivative (DTGA) to access their thermal degradation data in order to verify the efficacy of the addition of plasticizer in copolymer thermal stability. The onset decomposition temperature (T_D) was defined as that corresponding to 2% weight loss due to degradation; and peak decomposition temperature (T_P) was obtained from the maximum amount of DTGA. Table 4 summarizes the events observed, initial decomposition temperature (T_I), T_D, T_P, the organic material content, determined as the mass loss from 25°C up to 600°C, and stable residue content at 600°C.

For the pure copolymer sample, there was a standard mass loss that occurs in a single stage and in a narrow temperature range, initially with 263°C and ending with a final decomposition temperature of approximately 315°C. If compared to the main range of PHB decomposition (from 220°C to 250°C)^[10], it can be noted that although the PHBV decomposition range is also narrow, the copolymer has more thermal stability, characterized by the major temperatures presented. The additive formulations showed a minor mass loss corresponding to the additive decomposition, which occurs before the PHBV continuous mass loss. The 5P01N formulation shows a 5% mass loss, from 179°C up to approximately 280°C, related to the amount of plasticizer in the formulation. The 7P01N formulation lost the equivalent of 5% additives, while for the 10P01N sample, this loss was around 7%. This reduction in the amount of additives in each formulation indicates the exudation (or migration to the surface) of the plasticizer after the process. It is probable that the addition of the PHBV with the P-902 plasticizer is viable up to 6% additive, which would explain why the incorporation of more plasticizer to PHBV did not improve the mechanical properties of the formulations.

The temperature where the decomposition rate is at maximum stage varied with the formulations: the PHBV copolymer had a 303°C decomposition peak temperature; the 7P01N and 10P01N formulations were less stable, showing the lowest temperatures, 275°C and 281°C, respectively. It was observed that the 5P01N blend was the formulation with the greatest thermal stability, because the greatest decomposition rate occurred in the highest temperature (306°C), in which the effective mass loss starts at approximately 30°C, being above the other formulations.

3.3. Polarized Light Optical Microscopy (PLOM)

The effect of additives to the nucleating agent in the crystalline morphology of pure PHBV and PHBV compositions with 5% of plasticizer was studied through Polarized Light Optical Microscopy (PLOM). Figure 4 presents the PLOM images for isothermal crystallization at 60ºC. The first image corresponds to the isotherm initial time, while the second shows the morphology after 20 minutes of crystallization. In the images of the first column, small spherulites that grew from stable nuclei (non-visible) can be seen. At this temperature, the effect of the nucleating agent with additives in the formulations is expected and desired, i.e., it has a large amount of stable nuclei, resulting in smaller sizes of spherulites and more even distribution when compared to the pure copolymer. The formulation with the highest concentration of nucleate showed a more refined structure, suggesting that under these conditions the 5P05N formulation would have the best mechanical properties. On the other hand, at higher temperatures (80°C and 90°C), the formulations with additives had a lower amount of stable nuclei if compared to the pure formulation, indicating that under these temperatures no nucleating effect of Licowax occurred. At the same time, there was an effect of the plasticizer in restraining the formation of these nuclei. Comparing the images based on the crystallization temperature for each formulation, it is clear that the number of visible stable nuclei was reduced with the increase in the temperature, suggesting that under the PHBV melting temperature, Licowax also reached fusion.

Figure 5 presents the rate of spherulite formation for each isothermal crystallization temperature (60°C, 70°C, 80°C and 90°C) studied, obtained through the measurement of spherulite radius according to time. Immediately, it is verified that, for all formulations, the maximum increase rate occurred at 80^ºC, as already reported in other works²¹. It is also noted that the formulations with additives presented almost the same increase on the spherulite growth rate, superior than pure PHBV. The faster spherulite growth rate indicated that the additives had an effect on the PHBV crystallization kinetics. However, from these results, it can be attested that the formulations with Licowax were not so efficient in relation to the refining of the crystalline structure. The reduction of stable nuclei in the formulations with additives shows that this additive did not perform as a classical nucleating agent, a characteristic previously observed through DSC thermal analysis¹¹. Nevertheless, the efficient crystallization provoked by the presence of Licowax is noticed in the performance presented by the formulations containing this additive in relation to the mechanical properties.

3.4. Biodegradability evaluation

Test specimens of pure PHBV and PHBV with additives were exposed to biodegradation during 60, 120 and 180 days of organic compound, according to the Sturm methodology^15,16. Five test specimens for each formulation were weighed before being submitted to testing for subsequent calculation of their mass loss. These results are presented in Figure 6. In general, it was observed that the longer the test specimen exposure time in organic compound, the greater was the mass loss presented by them. The additive formulation had a greater mass loss than the pure copolymer, indicating that the chosen additives (P902 and Licowax) did not affect the polymer biodegradation, inclusively, accelerating the PHBV microbiological degradation. Given this finding, it is possible to affirm that the additives used in this work can accelerate PHBV biodegradation, probably due to the reduction of its crystallinity. Considering pure PHB biodegradation studies^16,21, it showed approximately 5% mass loss after 180 days, a value very close to the result reached in this work for PHBV without additives. Calculating the mass variation for each formulation in each removal (m_f - m_i) by the number of test days, it was possible to come to an average biodegradation rate of 3.4 mg/day and 8.7 mg/day for the pure PHBV and the PHBV with additive, respectively.

The presence of additives in PHBV resulted in an increase of approximately 5% in the result of the mass loss after biodegradation tests. The visual aspect of these biodegradable samples, presented in Figure 7, shows more evident alterations in the surface of the copolymer with additive.

In order to verify changes in the mechanical properties of the biodegraded specimen tests, stress in the rupture results are shown in Table 5. There was a gradual decrease in the mechanical properties with the increasing time of biodegradation. As we know, the microbiological attack occurs, initially, in the polymer amorphous phase with the production of small "blanks" that contribute to the breaking of the material with little or no deformation^22,23. It was observed that the formulation with additives suffered greater damage in the tensile mechanical properties than PHBV, which makes sense, since this formulation shows greater mass loss than the pure copolymer. Figure 8 presents the photomicrographs of pure PHBV and additive PHBV before and after biodegradation tests, where the structural changes that resulted from the attack of microorganisms are evident.

4. CONCLUSIONS

In general, the addition of PHBV copolymer with the plasticizer P-902 and the nucleating Licowax with different contents resulted in an improvement in the properties of the pure copolymer, characterized by the reduction of rigidity. Nevertheless, the increase in the amount of additives in the formulations did not make them more efficient. The increase in the nucleating content served as an inhibitor of the plasticizer action. The increase in the plasticizer percentage was not proportional to the increase of the properties due to its migration to the surface. The formulation with the minimum amount of additive content, 5% P-902 and 0.1% Licowax, was the most effective in adding additives, with the best results: 35% reduction in the elastic constant, and 18% in the PHBV crystalline degree; and also 58% increase in impact resistance and 46% increase in elongation. Furthermore, it is important to emphasize that the use of lower additive content has an effect on the lower cost in the final product.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge PHB Industrial S/A, and PHBV supply; and also the Brazilian research funding agencies CAPES and CNPq for the financial support and the scholarships.

Received: August 28, 2013

Revised: August 21, 2014

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Publication Dates

• Publication in this collection
  21 Oct 2014
• Date of issue
  Oct 2014

History

• Received
  28 Aug 2013
• Reviewed
  21 Aug 2014