Biodegradable Polymer

Biodegradable Polymer: Poly(butylene succinate)/Poly(butylene adipate-co-terephthalate) Clay Nanocomposites

Abstract: Sodium montmorillonite (Na-MMT) was successfully modified by octadecylamine (ODA) to be oganoclay through cation exchange technique which showed by the increase of basal spacing of clay by XRD. The addition of the organoclay in to the PBS/PBAT blends produced intercalated type nanocomposites. Tensile tests have shown that the stiffness increases continuously with clay loading which attributed to the existence of strong interactions between PBS/PBAT and nanofillers, particularly with organoclay. Highest tensile strength of nanocomposite was observed at 1 wt% of organoclay incorporated. TGA study showed that the thermal stability of the blend increased after the addition of the organoclay by 1 wt%. SEM micrographs of the fracture surfaces show that the morphology of the blend becomes smoother with presence of organoclay.

Keywords:

Biodegradable, organoclay, nanocomposite

INTRODUCTION

Packaging is by far largest market for plastics. Enormous and varieties of plastics used today are produced mostly from fossil fuels in particular polystyrene and poly(vinyl chloride) PVC. These plastics do not degrade spontaneous and thus treated by incineration, resulting in large amount of carbon dioxide gases and contributing to global warming. Packaging plastics are end up as municipal solid waste after discarded. Limited landfill sites and environmental concerns related to the use of conventional plastics have forced scientists to develop biodegradable polymer materials for packaging applications.

Biodegradable polymers are a degradable polymer in which the degradation results from the action of naturally occurring microorganism such as bacteria, fungi and algae. Some of the widely available biodegradable polymers are poly(lactic acid) (PLA), polyhydroxybutyrate (PHB), polyhydroxyalkanoates (PHA), poly(caprolactone) (PCL), poly(butylene succinate) (PBS), poly(butylene adipate-co-terephthalate) (PBAT), thermoplastic starch, cellulose and etc [1]. Among this, aliphatic polyesters are the most promising materials for the production of high performance packaging products [2]. However, in their practical uses, their low melting point, low thermal stability and higher cost compared to conventional plastics, limit their use.

Poly(butylene succinate) PBS (BIONOLLE #1020) is a biodegradable aliphatic thermoplastic which synthesized by polycondensation of 1,4-butanediol with succinic acid with many desirable properties including biodegradability, melt processability, and thermal and chemical resistance . It can be processed in the field of textiles into melt-blown, multifilament, monofilament, flat and split yarn [3]. However, other properties such as gas barrier properties, melt viscosity for further processing and so on, are frequently insufficient for various end-use applications [4]. Poly(butylene adipate-co-terephthalate) PBAT, trade name ECOFLEX® is an aliphatic/aromatic copolyester based on the monomers 1,4-butanediol, adipic acid and terephthalic acid. It is fully biodegradable polymer and resistance to water. It exhibit excellent compatibility to other biodegradable aliphatic polyesters or starch compounds. Typical applications are packaging films, agricultural films and compose bags.

To improve the properties of PBS, many researcher introduce organoclay to the PBS system [5-7]. Researchers also try to improve the properties of PBS by blending with other polymers [8]. Jacob et al. have studied the compatibility between PBS and PBAT. They reveals that this blend is immiscible but only one Tg was observed due to the close proximity of individual Tg's of both PBS and PBAT [9]. Other report have shown that organoclays can acts as compatibilizers for immiscible polymer blends [10-16] which exhibit remarkable improvement in their properties compared to the pristine polymer or the conventional composite [17-19]. Thus, in this study ODA-MMT (organoclay) will be used as compatibilizer to improve the miscibility of the PBS/PBAT blends. The PBS/PBAT blends will be prepared by melt blending technique and the effect of clay on physical, mechanical and thermal properties will be investigated. The compatibility between the two polymers will also be studied by examine the fractural surface of the tensile test under SEM.

EXPERIMENTAL

Materials

Poly(butylene succinate), PBS, was supplied by Showa Highpolymer Co. Ltd. (Japan) under tradename “BIONOLLE”. The PBS pellets were dried in oven at 60oC for 24 hours before processing. The structure of PBS is shown in Figure 1. The poly(butylene adipate-co-terephthalate), PBAT, trade name ECOFLEX®, F BX 7011 was supplied by BASF. PBAT was supplied in pellet form and used as received. Structure of PBAT is shown in Figure 2. Sodium montmorillonite (Na-MMT) (Kunipia F) with cation exchange capacity of 119 meq/100g was obtained from Kunimine Ind. Co. Japan and used as received. Octadecylamine was purchased from Acros Organics and used as an organic modifier of montmorillonite.

Preparation of organoclay

Organoclay ODA-MMT was prepared by adding a dispersed 20.00 g of sodium montmorillonite in 800 ml of distilled water at 80oC into a solution of 13.476 g octadecylamine and 4.81 ml concentrated hydrochloric acid in 200 ml hot distilled water. The resultant suspension was vigorously stirred for 1 hour. The precipitate was repeatedly filtered and washed with hot distilled water until no chloride ion was detected with 0.1M AgNO3 solution. It was then dried at 60oC for 24 hours. The organophilic montmorillonite was ground with a mortar and sieved into particles of size of 75 µm which will then used for the preparation of nanocomposites [20].

Preparation of Poly(butylene succinate) (PBS)/Poly(butylene adipate-co-terephthalate) (PBAT)/clay nanocomposites

Nanocomposites containing a thermoplastic blend and clay were produced by melt compounding. The blend composition was kept constant (PBS 70 wt % + PBAT 30 wt %), whereas the clay content was varied between 0 and 7 phr. PBS, PBAT and clay were manually premix in a container and fed into Brabender at 130oC and rotor speed of 50 rpm. The residence time was maintained at 10 minutes for all the preparation. The products were then compression molded into sheets of 1 mm thickness by an electrically heated hydraulic press with a force of 1500kN at 125oC for 10 minutes. The sample sheets were then use for further characterization.

X-ray diffraction (XRD)

X-ray diffraction measurement was carried out by using a Shimadzu XRD 600 X-Ray diffractometer with CuKα radiation (λ= 1.542Å) operated at 30 kV and 30 mA. Data were collected within the range of scattering angles (2θ) of 2 to 10o at the rate of 2o/min. The interlayer spacing of the clay was derived from the peak position (d001 reflection) in the XRD diffractogram according to the Bragg equation (λ=2dsinθ).

Fourier Transform Infrared (FTIR)

Fourier transform infrared spectra were recorded using a Spectrum BX Perkin Elmer in the range of frequency 280 cm-1 to 4000 cm-1 at 25oC.

Tensile Properties

Tensile properties test were carried out by using Instron 4302 series IX. The samples were cut into the dumbbell shape follow the ASTM D638 (type V) standard. Average thickness and average width of the gauge section of each specimen were calculated using three measurements of the thickness and width respectively. A digital micrometer, Mitutoyo model 293 705 (Japan) with an accuracy of 0.001mm was used in the measurements. The machine crosshead speed used was 10mm/min. Tensile strength, tensile modulus and elongation at break were evaluated from the stress-strain data. Seven specimens were tested and the average of the values was taken.

Thermogravimetric Analysis (TGA)

Thermogravimetric analysis was performed using a Perkin Elmer TGA7. The weight of the samples used was about 10mg and were heated from 35oC to 800oC at the heating rate of 10oC/min. The analysis was carried out in nitrogen atmosphere with nitrogen flow rate of 20ml/min. The weight loss of samples were recorded and plotted as the function of temperature.

Scanning Electron Microscopy (SEM)

Scanning Electron Microscopy was done using JEOL scanning microscope JSM-6400, Japan. Fracture surfaces were obtained from plain strain fracture tensile tested specimens. The fracture surfaces were sputter coated with gold.

Dynamic mechanical analysis (DMA)

Dynamic mechanical analysis was conducted according to ASTM D5023. The storage modulus (G′), loss modulus (G″) and mechanical loss factor (tanδ= G′/G″) as a function of temperature, were assessed using Diamond Perkin Elmer DMA. DMA spectra were taken in 3 point bending mode at 5 Hz frequency in a temperature range of -100oC to 100oC.

RESULTS AND DISCUSSIONS

Characterization of organoclay

Fourier Transform Infrared (FTIR)

FTIR spectra of ODA, Na-MMT and ODA-MMT are illustrated in Figure 3. For the unmodified montmorillonite, Na-MMT, the presence of broad band at 3400cm-1 indicates the presence of the free water molecules vibration. This peak indicates the hydrophilic nature of Na-MMT and the presence of water in the Na-MMT. The infrared spectrum of Na-MMT exhibited four unique characteristic peaks which correspond to the O-H stretching (3623 cm−1), Si-O stretching (996cm−1), Al-O stretching (516 cm−1) and Si-O bending (439 cm−1). The organoclay ODA-MMT presents three new peaks in the FTIR spectrum compared to Na-MMT. Bands at 2925 and 2853cm−1 are attributed to the C-H asymmetric and symmetric stretching vibrations of octadecylamine, respectively. The band at 1472cm−1 is assigned to the CH2 bending (scissoring) vibration. The existence of peak C-H stretching in organoclay confirm the intercalation of alkylammonium in the interlayer spaces of montmorillonite [21]. On the other hand, the organoclays also become more organophilic as shown by the reduction in intensity of free O-H stretching peak. Since the spectrum of ODA-MMT is a combination of spectrum Na-MMT and ODA, it indicates that ODA are intercalated into the silicate layer of Na-MMT, therefore the clay was successfully modified to become organoclay. IR spectrum of intercalate keeps all the characteristic absorption bands of the host structure Na-MMT.

X-ray diffraction analysis

The changes of basal spacing of the resulted materials can reflect the intercalation of surfactant octadecylamine into montmorillonite interlayer spaces [22]. The natural montmorillonite, Na-MMT has 1.24nm of basal spacing at 2θ=7.12o before it was modified with different surfactants. This basal spacing value is similar to the value obtained from the XRD measurement of previous work [23]. The values of 2θ and basal spacing of clays are summarized in the Table 1. The XRD patterns for the clays are illustrated in Figure 4. After being modified by the surfactant octadecylamine (ODA), the basal spacing of the clay increase from 1.24nm to 2.83nm. These indicated that the montmorillonite is successfully modified by the surfactant and organoclay is formed. The exchange cation is intercalated into the galleries of silicate layers after it is exchanged with the sodium ion.

Characterization of nanocomposites

Fourier Transform Infra (FTIR)

Figure 5 shows the infrared spectra of the PBS/PBAT and PBS/PBAT/clay composites. FTIR was carried out to study the effects of functional groups in the organoclay with respect to potential interactions with the blend constituents. The three spectra exhibited a strong absorbance peak at around 1714cm-1 which due to the vibration of carbonyl group, C=O. The C=O stretching slightly shifted to higher wavenumbers when clay was incorporated into the PBS/PBAT blend. This means that there has interaction between clay and C=O carbonyl group. The C-O stretching also observed at 1268cm-1. These are the characteristic peaks of PBS and PBAT. Note that there is an absorption band at around 1040 cm−1 was observed in nanocomposites. This band is characteristic of sodium montmorillonite (Na-MMT) [24]. This may due to the cation exchange capacity of the montmorillonite used was not fully exploited [25]. The Si-O and Al-O stretching peaks also observed in the nanocomposite which indicated the present of clay. The other peaks present in the spectrum are as same as the characteristic peaks of PBS/PBAT spectrum. Therefore, there is only physical interaction and no new bond is formed.

X- ray diffraction analysis

Figure 6 shows the XRD pattern of Na-MMT and PBS/PBAT/Na-MMT composites at different clay loading. Due to the hydrophilic nature of Na-MMT, it is hardly intercalated into the polymer blend. The basal spacing of composites with the clay loading of 1 wt% and 3 wt% could not be detected, which may be due to low clay content or absence of any ordered layer structure. This lack of intergallery clay diffraction is due to the exfoliation and random distribution of the clay platelets within the polymer blend [26]. When 5 wt% and 7 wt% of Na-MMT is added to the polymer blend, the diffraction angle of Na-MMT is shifted from 7.12o (1.24nm) to lower angle 6.48o (1.36nm) and 6.58o (1.34nm) respectively. The increase in the basal spacing of the composites compared with that of the corresponding neat clay indicates that the PBS/PBAT chains were intercalated into the clay matrix during the direct melt blending process. The presence of peak of the clay in the composite indicates the intercalation of the clay structure and in turn confirms the formation of composites.

The intercalation of the polymer chains would increase the interlayer spacing compared with that of clay, which would shift the peak towards a lower angle. Figure 7 shows the XRD pattern of ODA-MMT and PBS/PBAT/ODA-MMT nanocompoistes. ODA-MMT show s a sharp peak at diffraction angle of 3.12o corresponding to 2.83nm of basal spacing. The increases in the basal spacing of ODA-MMT from 2.83nm to 3.59nm, 3.45nm, 3.44nm and 3.31nm when 1 wt%, 3 wt%, 5 wt% and 7 wt% of ODA-MMT respectively incorporated into PBS/PBAT, indicated that the polymer chain is successfully intercalated into the clay layers forming intercalated type nanocomposites. The intensity of peak became higher as the content of ODA-MMT in PBS/PBAT/ODA-MMT composite increased and the interlayer spacing shirted towards a higher diffraction angle. For example, the diffraction angle of PBS/PBAT/1 wt% ODA-MMT appeared at 2.46o (3.59nm) while the diffraction angle of PBS/PBAT/7 wt% ODA-MMT appeared at 2.67o (3.31nm). The existence of sharp peaks shows that all the nanocomposites still retain an ordered structure. Therefore, it can be concluded that the well ordered intercalated nanocomposite were formed after melt mixing.

Transmission Electron Microscopy

The dispersion of layered silicates in the composites were observed by TEM as shown in the Figure 8, which presents PBS/PBAT composites with 1wt% of (a) Na-MMT and (b) ODA-MMT. The dark lines are the cross section of intercalated silicate layers and bright areas are the matrix.

In PBS/PBAT/Na-MMT, the clay persists as tactoids of agglomerates throughout the polymer matrix as shown in Figure 8(a). This incomplete dispersal of the reinforcing phase inhibits ideal surface contact between the polymer and clay, creating large regions of clay tactoids in the composite. This observation is in agreement with the XRD result which shows little shifting in the diffraction peak of Na-MMT for PBS/PBAT/Na-MMT composites and formed as immiscible or phase separated micro-composites.

On the other hand, PBS/PBAT/ODA-MMT shows that the layers of clay were disordered and the PBS/PBAT matrix was intercalated into the gallery of the clay. It is consistent with the XRD result shows that a large increase in basal spacing of the silicate layers and formed intercalated type of nanocomposites.

Tensile Properties

Figure 9 shows the effect of clay loading on the tensile strength of PBS/PBAT blend. The tensile strength of pristine PBS/PBAT blend is 21.20MPa. The addition of hydrophilic clay, Na-MMT decreases the tensile strength to 19.76MPa. This is due to the hydrophilic nature of montmorillonite which is incompatible with the hydrophobic polymer matrix.

However, tensile strength is improved by 13.5% to 24.06 MPa by addition of organoclay, ODA-MMT at low clay loading (1 wt%). This suggests that ODA-MMT act as reinforcing filler, which increase interaction at the phase boundaries upon the addition of organoclay. Strong interphase interaction is believed reduce the stress concentration point when tensile load is applied. The organoclay is able to act as reinforcing filler due to its high aspect ratio and platelet structure. When small amount of clay (1 wt%) compounded into polymer matrix, clay is located in the interphase between the matrix and the dispersed phase. However, when the amount of clay is above 1wt%, only a part of the clay locates in the interfacial area, and the excess is dispersed in the matrix affecting its homogeneity and consequently the tensile strength of the blends [27, 28]. Thus, further addition of ODA-MMT leads to the reduction of tensile strength. As the clay loading increased above 1 wt%, the tensile strength decreases for both clays. This may be due to poor dispersion or agglomeration.

The tensile modulus of PBS/PBAT nanocomposites are shown in the Figure 10. The figure shows that tensile modulus of all nanocomposites increase with the increasing of the clay loading [29]. Montmorillonite silicate has been found to be efficient in stiffening polymers [30]. The tensile modulus of PBS/PBAT/ODA-MMT increases 25.7% from 200.46 MPa to 269.90 MPa, whereas the tensile modulus of PBS/PBAT/Na-MMT increases 16.1% to 238.8 MPa. The increase in tensile modulus is mainly due to the high aspect ratio and rigidity of clay layers. However, tensile modulus increase in PBS/PBAT/ODA-MMT is higher than PBS/PBAT/Na-MMT at all clay loading. This indicates that organoclay is more compatible with polymer blend compared to Na-MMT, and hence increase the stiffness of the nanocomposites. Once again the organoclay act as reinforcing filler better than unmodified clay, Na-MMT. The enhancement of tensile modulus was reasonably ascribed to the constraint of the polymer chains by their interaction with the clay surfaces [31].

The increase in modulus by incorporation of clay in the polymer matrix always sacrifices the elongation at break, which can show from the Figure 11. The decrease in the elongation at break can be explained by the existence of aggregated clay structures which increase the brittleness of the nanocomposites. Furthermore, the movement of polymer matrix is restricted by interfacial interaction between clay and polymer matrix, thus lead to the reduction in elongation at break.

Thermogravimetric analysis

Generally, introduction of clay into polymeric matrices can improve their thermal stabilities since the montmorillonite can hinder the permeability of volatile degradation products out of the materials. The dispersed clay generates a barrier which delays the release of thermal degradation products in comparison the pristine polymer [23]. Figure 12 and 13 show the TGA and DTG thermogram respectively of PBS/PBAT blend and PBS/PBAT with different clays. The weight loss of the blend and nanocomposites due to degradation is monitored as a function of temperature. The characteristic thermal parameters selected were onset temperature, which is the initial weight loss temperature, and maximum degradation temperature, which is the highest thermal degradation rate temperature. The results are summarized in the Table 4. The PBS/PBAT blend show onset temperature of 215.33oC, which increased to 223.60oC and 237.48oC when Na-MMT and ODA-MMT clays respectively incorporated into the blend. The incorporation of clay into polymer matrix was found to enhance the thermal stability. The improved thermal stability attributed to an ablative reassembling of the silicate layers which may occur on the surface of the nanocomposites creating a physical protective barrier on the surface of the material which hinder the diffusion of volatiles and assist the formation of char after decomposition [32]. However, the organoclay ODA-MMT give higher thermal stability compare to the hydrophilic Na-MMT indicating better interaction between PBS/PBAT matrics and organoclay. The maximum degradation temperature (Tmax) also increased by the compounding the PBS/PBAT with organoclay.

Dynamic mechanical Analysis (DMA)

One of the aspects of thermomechanical behaviour required for high performance applications is the ability of a polymeric material to withstand load at elevated temperatures. Dynamic mechanical analysis is a method in which the elastic and viscous response of a sample under oscillating load, are monitored as a function of temperature. During a heating at constant frequency, the storage modulus, G′, usually strongly decreases when temperature crosses the dynamic glass transition (α relaxation). On the other hand, the loss modulus, G″, and the loss factor exhibit a peaked shape. Dynamic mechanical analysis for the PBS/PBAT clay nanocomposites was carried out to see the effect of the organoclay on the thermomechanical properties

The dynamic storage modulus for the PBS/PBAT and PBS/PBAT/clay are shown in Figure 14. All the curves show the same pattern, and they can be divided in three main zones: glassy (from -100 to -30 °C), glass-rubber transition (from -30 to 50 °C) and rubbery (from 50 to 100 °C). In the first two zones, the storage modulus (G′) of the PBS/PABT increased with the incorporation of clay [33]. This is in agreement with the tensile modulus from tensile tests. This means that the clay has a considerable effect on the elastic behaviour of the nanocomposite. The presence of ODA-MMT in nanocomposites greatly improves the stiffness of PBS/PBAT blend leading to a higher storage modulus. Jian et al. showed that the changes of G′ reflect the effect of clay dispersion [34]. Organoclay are well dispersed in PBS/PBAT matrix and give increases in the stiffness of the composite. The increase of the storage modulus can be attributed to maximizing the adhesion between the polymer matrices and clay surfaces because of nanometer size, which restricts segmental motion near the organic-inorganic interface. This result of DMA analysis suggests the improvement in the thermal properties by incorporation of clay.

Figure 15 shows loss modulus of PBS/PBAT and it composites. The enhancement of G″ is much higher in the PBS/PBAT/ODA-MMT compared to PBS/PBAT/Na-MMT. The peak intensity of G″ represents the melt viscosity of the polymer. This means that the incorporation of organoclay enhaces the melt viscosity of the corresponding nanocomposite. The silicate platelets of high aspect ratio are well separated and this strongly increases the viscosity of the melt [35]. Viscosity increase by organoclay dispersed has been attributed to the expansion and delamination of clay layers and structure formation between the layers due to strong hydrogen bond interactions which restrict the movement of polymer chain [36].

The tanδ peak has been used to investigate the glass transition of semi-crystalline polymers and blends or polymeric networks. The temperature dependence of tan δ of PBS/PBAT and PBS/PBAT nanocomposites are presented in Figure 16. The tan δ curves show two dynamic relaxation peaks at -30oC and 50-100oC, which referred to as α and β relaxation peaks respectively. The α relaxation peak is believed to be related to the breakage of hydrogen bonding between polymer chain which induces long range segmental chain movement in the amorphous area. The β-relaxation peak is segmental carbonyl groups in the amorphous area which do not involve in hydrogen bonding [37]. This first relaxation peak at -30oC assigned to the glass transition temperature, Tg, of the composite. The tan δ peak which corresponding to the values of Tg slightly shifted towards the lower temperature by the addition of clay compared to the PBS/PBAT blend. The presence of organoclay does not lead to a significant increase of Tg as compared to PBS/PBAT blend. This behaviour has been ascribed to the restricted segmental movements at the organic-inorganic interface neighbourhood of intercalated PBS/PBAT nanocomposite. However, the decrease of Tg of nanocomposites probably is an effect of plasticization due to intercalant molecule (ODA) in layered silicates [38]. As the glass transition process is related to the molecular motion, the Tg is considered to be affected by molecular packing, chain rigidity and linearity too. Since the height of relaxation peak is associated with molecular mobility, it has been observed that the incorporation of clay into polymer matrix reduces their molecular mobility [39]. This is noted by the slightly reduction in the height of tan δ peak. Thus, the reduction is a result of polymer chains confined between the clay interlayer.

Scanning Electron Microscopy (SEM)

The fracture surface of the nanomposites was examined by scanning electron microscope to study the morphology of the surface and the dispersion phase. Figure shows SEM micrographs of fractured surface of PBS/PBAT blend and its nanocomposites at magnification of 1000X. If the components have different melt viscosities, the morphology of the resulting blend will show finely dispersed phase of the component with lower viscosity. Higher viscosity will lead to coarse dispersion of phases in a spherical domain. However in the case of blends containing higher concentrations of PBS, a reversal of the continuous phase was visible [9]. Thus for PBS/PBAT blend, PBAT with higher viscosity formed the continuous phase as shown in the Figure 17(a). Some small void still could be observed in PBS/PBAT/Na-MMT (Figure 17(b)) compared to pristine PBS/PBAT which indicates that the incompatibility of Na-MMT with the PBS/PBAT matrix. The presence of organoclay influences the morphological structural changes of the blend. As shown in Figure 17(c), the PBS/PBAT phases have affinities to make nanolevel incorporation with organoclay. As a result, the nanocomposite showed more homogeneous and single phase morphology compared to unmodified clay. In other word, the organoclay is more compatible with the polymer matrix.

CONCLUSION

Organoclay (ODA-MMT) was successfully prepared from Na-MMT via cation exchange method by using octadecylamine as surfactant. Intercalated type of PBS/PBAT/ODA-MMT nanocomposites was successfully prepared by melt blending as proved by XRD and TEM. Tensile strength of PBS/PBAT/ODA-MMT was improved by reinforcing effects of organoclay and gives maximum value at 1 wt% ODA-MMT loading. Thermal stability was enhanced by incorporation of clay into PBS/PBAT matrices. Storage and loss moduli were slightly increased with organoclay. SEM micrographs showed that presence of clay in the PBS/PBAT blend changed the morphology of the compound. SEM micrographs also support the finding of compatibility for the PBS/PBAT blend with organoclay through the homogeneous distribution for inorganic particles which gives an improvement in mechanical properties.

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