Polypropylene (PP) is an outstanding polymer material and used mainly in flexible packaging applications such as food packaging, trash bags, etc. due to its low cost and excellent physical, mechanical, and thermal properties [1, 2]. But, after their usage period, it is carelessly discarded in open dumps, landfills, or as simple litter. And, owing to its large molecular weight, hydrophobicity, and non-biodegradable nature, it is hard to be metabolized due to which it accumulates in land and water bodies . A rapid development of researches on blending of non-biodegradable polymers with the biodegradable ones to enhance biodegradation has been witnessed during the last few decades [4, 5]. However, most of these studies have been done using starch or cellulose for introducing biodegradability in polyolefin, which results in degradation of starch/cellulose, only leaving behind the polyolefin part. Moreover, blends with starch/cellulose do not show good mechanical properties . In fact, the performance of polymer blends depends on the composition of ingredients, their properties, and their morphology .
Blends of PP with poly(L-lactide) (PLLA) have been studied earlier to improve dyeability of PLLA , melt processability of PLLA , and rheological and mechanical properties . PP and PLLA form incompatible blends, and a compatibilizer is needed to improve compatibility between them. Maleic anhydride grafted polypropylene (MAPP) is a well-known compatibilizer used for PP-based blends and composites. It follows the free-radical mechanism – the anhydride part of MAPP has free radical site at α-carbon of carbonyl group, which forms ester linkages with PLLA, and the PP part of MAPP shows compatibility with PP matrix . In this way, it acts as a bridge and helps in improving the interfacial adhesion between PP and PLLA.
A number of efforts have been done to study the degradation behavior of PP and increase the same through various means [10–15]. It can be achieved if somehow PP is degraded to low-molecular-weight compounds so that microbes can easily attack the polymer surface . Addition of PLLA into PP will make its surface hydrophilic to some extent, which helps in accelerating degradation process by the microorganisms. PLLA is a well-known biodegradable polymer and has high strength, modulus, and biocompatibility. It is widely used for various applications like biomedical devices, tissue engineering, etc. It has potential to resolve the current problems concerning global warming on account of its carbon neutrality. However, its brittleness and low heat distortion temperature limit the application of neat PLLA [17–19]. Some of the previous studies also reported that Pseudomonas stutzeri bacterium shows potential to form biofilm, i.e., attachment of microorganisms to polymer surface, on the surface of PP [20, 21].
The purpose of our study was to develop partially biodegradable polypropylene films which should retain their mechanical and other requisite properties for flexible packaging applications during their lifespan and then undergo effective biodegradation after their useful life. In the present study, PLLA was blended with PP, with/without compatibilizer, in different ratios, and their mechanical, physical, and morphological characterizations were performed through various techniques. Biodegradation of the films prepared from PP/PLLA blends was also analyzed using bacteria P. stutzeri in laboratory conditions.
2 Materials and methods
The polymers used in this study, PP, PLLA, and the compatibilizer (MAPP) were of commercial grade whose sources and properties are mentioned in Table 1. Pseudomonas stutzeri (MTCC number 2643) was procured from Microbial Type Culture and Gene Bank, Institute of Microbial Technology (IMTECH, Chandigarh, India). It is a Gram-negative, motile, and rod-shaped bacterium having an optimum growth at 35°C. Nutrient broth (NB, in liquid media) and nutrient agar (in solid media) were obtained from HiMedia Laboratories Ltd. (Mumbai, India). For degradation studies, synthetic medium containing (g/l of distilled water): 1.0 g NH4NO3, 0.7 g MgSO4·7H2O, 1.0 g K2HPO4, 1.0 g KH2PO4, 0.005 g NaCl, 2 mg/l each of FeSO4·6H2O and ZnSO4·7H2O, and 1 mg/l of MnSO4·H2O was used. All these chemicals were obtained from HiMedia Laboratories Ltd. (Mumbai, India).
2.2 Blend preparation
Blends of PP/PLLA, with and without compatibilizer MAPP, were prepared in a Brabender Plasticorder PLE-651 mixer by keeping rotor speed of 60 rpm. The fill factor was 0.8, and mixing was carried out at 180°C for 4 min. The extrudate thus obtained was sheeted out through a two-roll mill set at room temperature with a nip gap of 2 mm, and the films were casted in a compression molding hydraulic press (George E. Moore Press, UK) electrically heated at 180°C for 2 min at a pressure of 3 MPa. While molding, Teflon sheets were used to prevent the molten mass from sticking to the mold. To maintain the overall dimensional stability of the samples, moldings were allowed to cool under pressure by circulating cold water through the platens. Neat PP and neat PLLA films were also prepared in similar fashion. The composition of prepared films is described in Table 2.
3 Testing methodologies
3.1 Mechanical properties
Tensile strength at yield, elongation at break, and elastic modulus were measured according to American Society for Testing and Materials (ASTM) D 882-91 standard on a universal testing machine (Zwick Roell Z010, Germany) with a clamp separation of 100 mm. The crosshead speed was kept at 50 mm/min, and the test was conducted at room temperature. Three specimens were tested for each composition, and the average values were taken.
3.2 X-ray diffraction
X-ray diffraction (XRD) spectra were obtained at room temperature using a Philips X’Pert diffractometer (Almelo, The Netherlands) with monochromatic CuKα radiation (λ=1.5418 Å) operated at 45 kV and current 40 mA. The scanning speed and diffraction angle (2θ) were kept at 5°/min and 5–60°, respectively. From XRD analysis, percentage of crystallinity (χ) and crystallite size (P) was measured according to Equations (1) and (2), as follows :
where Ic and Ia are integrated intensities corresponding to crystalline and amorphous phases, respectively.
where β is half width (in radian) of the crystalline peak, λ is the wave length of the X-ray radiation (1.54 Å), and k is the Scherer constant having a value of 0.9.
3.3 Fourier transform infrared spectroscopy
The interactions between PP, PLLA, and MAPP were studied by Fourier transform infrared spectroscopy (FTIR) analysis in attenuated total reflectance mode. It was carried out using FTIR spectrophotometer (Cary 600 Series, Agilent Technologies, USA) within the wave number range of 400–4000 cm-1. A total of five scans were accumulated at a resolution of 4 cm-1. The spectra were analyzed using Agilent Resolution Pro software.
Scanning electron microscope (JSM-6510LV, JEOL, Japan) was used to examine the surface morphology of polymer films. The specimens were coated with gold in an automatic sputter coater (JFC1600) to avoid charging effect and to enhance emission of secondary electrons. The instrument was operated at 10 kV.
3.5 Biodegradability studies
Biodegradability studies were performed using pure culture in synthetic media. The culture (P. stutzeri) was in freeze-dried form in a sealed depressurized ampoule, which was broken carefully with sterile file in laminar air flow (Thermodyne Pvt. Ltd., India), and 300 μl of sterilized NB was added for suspension preparation. NB was sterilized by autoclaving at 121°C and 15 psi pressure for 15 min. The prepared suspension was added to 250 ml Erlenmeyer flasks containing 100 ml NB and incubated for 24 h at 35°C and 120 rpm in a shaker incubator (New Brunswick Scientific, USA). This grown culture was used as inoculum in biodegradation studies. Synthetic media (100 ml) was placed in 250 ml conical flasks, and the film samples (2×2 cm) were added in duplicates and inoculated with 5 ml of freshly grown culture. Prior to degradation, polymer films were weighed, disinfected in 70% ethanol, and dried under laminar air flow. The flasks were incubated in the shaker incubator at 35°C and 120 rpm for 60 days. Polymers (PP100, PP80, PP80C6, and PLLA100) were added as sole source of carbon and energy in synthetic media. Biodegradation was monitored in terms of weight loss, optical density (OD) of bacteria, and thermogravimetric (TG) analysis. OD at 600 nm was taken in UV-visible spectrophotometer (Lambda 35, Perkin Elmer, USA) after every 5 days to verify the formation of biofilm on polymer surfaces. Weight loss was measured after 60 days of incubation in the media. Prior to weight measurement, biofilm was removed by washing the polymer film samples with 2% sodium dodecyl sulfate for 4 h and subsequently with distilled water. The washed samples were dried overnight in an oven at 60°C and then weighed.
The TG and derivative TG (DTG) analysis of the polymer films before and after aging was determined by using TG analysis (Q-500, TA Instruments, USA) under a nitrogen flow of 50 ml/min. Samples weighing 10±2 mg were heated from room temperature to 600°C at a heating rate of 20°C/min.
4 Results and discussion
4.1 Mechanical properties
The mechanical properties (tensile strength, elongation at break, and tensile modulus) of the PP/PLLA blends are shown in Table 3. As observed, neat PP had tensile strength of 45.4 MPa and tensile modulus of 941 MPa, which decreased on addition of PLLA due to its brittle nature. The increase in amount of PLLA caused the decrease in tensile strength, and the blend of 50:50 had the lowest strength amongst all the blends. In an ideal or highly compatible polymer blend, the blends are expected to have strength higher than or at least in between that of the pure polymers . In the case of PP/PLLA, the blends had lower strength than neat polypropylene. With the addition of 20% PLLA, tensile strength reduced to 31.3 MPa. But on further increasing the proportion of PLLA, i.e., up to 30%, the tensile strength decreased drastically to 19.83 MPa, which is only 46% of neat PP. The elongation at break (Eb) showed zig-zag trend. The blend of 80:20 showed almost similar value of Eb as of PP, but further addition of PLLA led to continuous decrease in Eb. This trend of tensile strength and elongation at break in graphical form is shown in Figure 1. The tensile modulus showed decrease in its value with increase in content of PLLA, but it was quite higher in the case of PP80 and PP70. Therefore, PP80 (80% PP+20% PLLA) was chosen as optimum considering the better retention of mechanical properties as well as the relatively low proportion of expensive PLLA.
The effect of adding compatibilizer from 2 phr upto 8 phr was investigated for the blend containing 80% PP. The addition of compatibilizer increased the tensile strength and tensile modulus than pure PP but decreased the value of Eb. This increase in tensile strength and modulus can be attributed to fine bonding and formed interaction between PP and PLLA molecules with the addition of compatibilizer. The changes in tensile strength and elongation at break by varying MAPP composition in PP80 blend is shown in Figure 2. So 6 phr of compatibilizer in the blend was considered as optimum amount by which the blend showed better tensile properties.
Hence, PP80 (without compatibilizer) and PP80C6 (with 6 phr compatibilizer) were chosen for further studies and characterized and compared with the neat polymers.
4.2 X-ray diffraction
The XRD patterns of the selected film samples are shown in Figure 3. It was observed that PP100 showed prominent peaks at 2θ=14.1°, 16.8°, 18.6°, and 21.8°, which corresponds to the α-monoclinic form of polypropylene, and PLLA100 showed major peaks at 2θ=16.7° and 19°. This has been reported in literature also [2, 8]. After addition of PLLA into PP as in the case of PP80 and PP80C6, the spectra matched very closely to neat PP. This indicates no or minimum change in crystal lattice dimensions of blends as compared with neat polypropylene.
The percentage of crystallinity of different polymers and crystallite size has been shown in Table 4. The addition of PLLA into PP causes increase in percentage of crystallinity from 79.7% to 86.7%, whereas compatibilized blend possesses almost the same crystallinity as of pure PP. Pure PP and PP80C6 have the same crystallite size (337 Å), whereas the crystallite size of PP80 (289 Å) is intermediate of pure PP and PLLA. This investigation showed that MAPP improved compatibility between the polymers.
4.3 FTIR studies
FTIR was carried out to study the occurrence of any chemical reaction between PP and PLLA. Figure 4 depicts the FTIR spectra of neat PP, neat PLLA, PP/PLLA, and PP/PLLA/MAPP blends. PLLA100 showed peaks at 1746, 1178, and 1084 cm-1, which were associated with C=O stretching, C-O-C symmetric stretching, and asymmetric CH3 bonds, and these peaks were absent in pure PP. PP100 showed C-H stretching, C-H bending, CH3 bending, and CH2 wagging corresponding to 2915, 1450, and 1367 cm-1, respectively. The spectrum of PP80 showed transmittance peaks around 2915, 1746, 1450, 1367, 1191, and 1084 cm-1, corresponding to both PP and PLLA indicating the absence of chemical interactions between the two components. Thus, there was decrease in mechanical properties of PP80 as compared with PP100. But addition of MAPP into the blend resulted in emergence of a new peak at 1757 cm-1 representing the carbonyl of ester linkage stretching. This indicates the interaction of MAPP with the PP/PLLA blend, which supports the XRD and tensile testing results. FTIR studies also showed a little chemical interaction between the two components after adding MAPP, which were not observed in the blend not containing the compatibilizer.
4.4 Morphological analysis
Mechanical properties of blends largely depend on morphology; therefore, scanning electron microsopy (SEM) analysis was carried out to better understand the correlation with morphology and structure of blends. The surface images of PP100, PP80, PP80C6, and PLLA100 are shown in Figure 5A, B, C, and D, respectively. It was noticed that the surface of PP100 is smooth and that of PLLA100 is rough that contains some cracks and pits due to its brittle nature. The surface morphology of the blend PP80 showed dispersion of PLLA in PP matrix, but phase separation and boundary layer between the two components could be seen clearly. It depicts low interfacial adhesion and immiscibility between the two components. It was due to this that the addition of PLLA caused reduction in tensile strength of PP. However, addition of MAPP introduced compatibility between PP and PLLA in their blend, and as shown in the micrograph of PP80C6, PLLA was almost uniformly dispersed in the PP matrix, which resulted in increase in the tensile strength of PP80C6 sample, as described earlier. Therefore, it may be concluded that MAPP has improved the interfacial adhesion between PP and PLLA due to reaction between PLLA functional group and the anhydride group present in MAPP .
4.5 Biodegradability studies
4.5.1 Weight loss
The weight loss in polymer samples after 60 days of incubation was found to be 1.21%, 1.67%, 2.41%, and 3.01% for PP100, PP80, PP80C6, and PLLA100, respectively. PLLA100 showed the maximum weight loss. As the surface of PLLA is hydrophilic, more microorganisms attached to its surface and caused effective degradation, while in the case of PP, microorganisms took some time to form a biofilm on the surface; therefore, least degradation was observed in PP. This observation indicates that the bacterial strain P. stutzeri has noticeable potential to degrade the blends when kept in media.
4.5.2 Optical density
The survival of bacteria in the presence of polymers was monitored by determining the growth in synthetic media. The OD of the synthetic media at 600 nm was observed every 5 days incubation period and is shown in Figure 6. The maximum growth was found in the case of PLLA, indicating that the microorganisms grow faster in media with PLLA as carbon source. In the case of PP, although OD increased, the extent was very much less as compared with PLLA. The OD for PP80C6 was found to be large enough, almost closer to PLLA. These data comply with weight loss data and show that PP/PLLA blends show more effective degradation than PP alone, and bacteria were able to utilize polymer samples as sole source of carbon which resulted in their partial degradation.
4.5.3 Thermogravimetric analysis (TGA)
The TG and DTG curves of polymers before and after aging are illustrated in Figures 7 and 8, respectively. As observed, TG curves of both PP100 and PLLA100 showed one-stage degradation, but PP/PLLA blends showed two-stage degradation. Two distinct peaks at 320°C and 390°C were identified in the TG curves of blends (PP80 and PP80C6) corresponding to the thermal degradation of PLLA and PP, respectively, which indicated partial compatibility between the blend components. Furthermore, it was found that the thermal stability of PP80C6 was more than PP80, which means that compatibility has been improved by adding MAPP into the blend. The DTG curves showed two maxima for blends whereas single maxima for PP100 and PLLA100, indicating that there is phase separation between the two components. After aging, the maxima shifted toward lower temperature due to decrease in thermal stability of the samples.
The initiation temperature (Ti), final temperature (Tf) and percent weight loss are shown in Table 5. The initial degradation temperature (Ti) corresponds to 1% weight loss of the polymer sample, and the final degradation temperature (Tf) corresponds to 1% residual left after which no appreciable loss is possible . The weight loss in virgin PP100 started at 319°C (Ti) and reached maximum at 478°C (Tf). This polymer sample, after exposure to bacterial aging for 60 days, shows 9°C decrease both in Ti and Tf, which indicates that it was not degraded much. But in the case of PP 80 sample, decrease in Ti was 22°C and that in Tf was 8°C. Similarly, 18°C decrease in Ti and 27°C decrease in Tf was noted in PP80C6, while in the case of PLLA100, 16°C decrease in Ti and 18°C decrease in Tf was observed. These data reveal that thermal stability of polymers reduced after aging in media, indicating considerable degradation of polymers. Maximum reduction in thermal stability was found in PP80C6 film sample.
Blends of PP/PLLA, with and without compatibilizer, were prepared in an internal mixer, and those blends were given the shape of films by compression molding technique. The ratio of PP/PLLA was optimized (separately for those with compatibilizer and without compatibilizer) on the basis of their mechanical properties. PP80 (80% PP and 20% PLLA) and PP80C6 (80% PP, 20% PLLA, and 6 phr MAPP) were found to be the best possible compositions, and, therefore, evaluated further for their physical, chemical, and biodegradation behavior. The physical and chemical properties showed that PP and PLLA form immiscible blend due to poor interfacial adhesion and polarity difference between them. This was also supported by mechanical strength data which showed decrease in mechanical strength of blends with increasing amount of PLLA. By adding 6 phr compatibilizer into the blend, improvements in mechanical strength, compatibility, and thermal stability were observed. The same indication was given by the XRD results. Biodegradation studies with P. stutzeri shows that PP/PLLA blends undergo more biodegradation than neat PP, and PP80C6 showed maximum degradation. Hence, these partially biodegradable blends of PP and PLLA could be used for some flexible packaging applications replacing neat PP, thereby reducing plastic pollution to a certain extent.
The authors are grateful to SAI Labs, Thapar University for carrying out SEM and XRD of polymers. We are also thankful to the Council of Scientific and Industrial Research (CSIR), scheme no. 02(0035)/11/EMR-II and TEQIP-II project for providing financial support to carry out the research work.
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