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Bio-polyester-date seed powder composites: morphology and component migration.

INTRODUCTION

The research focus on biopolymers has seen growing interest in the recent years because of the enhanced property profiles of these polymers as well as because of the risks of depletion of fossil fuels which provide the raw materials for most of the conventional polymers [1). Growing legislative constraints on the recycling and environmental dumping are also leading to the change in focus towards "green" or biomaterials. In order to enhance their properties and hence to generate high value materials from biopolymers, their composites using a variety of fillers like silica, layered silicates, nanotubes, etc., have also been reported [2-4], This leads to enhanced potential of biopolymers in replacing conventional polymers, especially thermoplastics, for a large number of commercial applications. However, a decrease in the rate of biodegradation of the polymer in the presence of fillers has occasionally being reported [5], thus, negating the positive effect of filler on other properties. By replacing the conventionally used inorganic fillers with bio-fillers, one can achieve "true" bio-composites which would also retain the biodegradable characteristics of the polymer. Though bio-filler may not be able to compete with the conventional fillers in terms of property enhancement, however, it is still expected to be functional in nature which brings significant enhancement in the polymer properties [6],

Date seed powder (DSP) represents one such bio-filler which can be used to functionally reinforce biopolymers. There is also a need of waste utilization because of the production of large amount of date fruit in the Gulf region which leads to the generation of significant amount of seed waste. Thus, the use of seed powder for the composite generation has a potential of not only adding value to the polymers, but also presents environmentally friendly routes for waste utilization. Some literature studies have reported the use of DSP for reinforcing polymers and described the resulting composite properties. Composites of high-density polyethylene (HDPE) and polystyrene were reported using locally produced date pits "khlaas" and "sekari", which were wastes of two types of date palm fruit grown in Saudi Arabia [7], Reduction in mechanical properties of composites was observed and was attributed to the coarse morphology of the date pit powder, especially in the absence of an appropriate coupling agent system. Ghazanfari et al. [8] also reported the incorporation of date seed flour in HDPE. The melt flow index of the composites was decreased, whereas the thermal conductivity of the composites increased. Various bio-composites involving other biofillers and biopolymers have also been reported. Chun et al. [9] reported the poly-L-lactide (PLA) eco-composites with coconut shell powder. The composites with untreated coconut shell powder were observed to have decreased tensile strength and elongation at break. Chun and Husseinsyah [10] also reported PLA composites with corn cob as biofiller. The addition of com cob resulted in decreased tensile strength and elongation at break. The authors reported that coconut oil coupling agent was needed to improve the mechanical performance of the composites. Wang et al. [11] reported the PLA composites with wood flour and the composites had a decrease in tensile strength from 55 to 27 MPa at 30% filler content. Averous and Digabel [12] reported poly(butylene adipate-co-terephthalate) (PBAT) composites with lignocelluloses-based fillers. The tensile modulus was observed to increase more than 3^4 times than the pure polymer at 30% filler content. Similarly, Siyamak et al. [13] reported PBAT composites with oil palm empty fruit bunch fibers, but the enhancement in the modulus was not as significant as reported for lignocellulose filler. A maximum increase of 23% in the modulus was observed in the presence of coupling agent.

It is of importance to further explore various aspects of biopolymer-DSP composites such as filler dispersion and composite morphology. Apart from that, the changes in morphology as a function of time are also required to be studied as these characteristics in bio-composites have significant time dependency. The presence and effect of date seed oil, if generated during the composite processing, is also needed to be confirmed [14], Understanding of these aspects is required to correlate the obtained composite properties with the constituent interactions and processing conditions in order to optimize the composite performance. In the current study, these aspects were studied using microscopy (optical, electron, and atomic force microscopy), shear rheology, spectroscopy, and X-ray diffraction. The bio-polyester matrices chosen for study were rubbery (PBAT) and brittle (PLA) at room temperature so as to explore the effect of date seed filler on their properties. The fraction of DSP in the composites was varied and time and temperature dependent studies were performed.

EXPERIMENTAL

Materials

PBAT bio-polyester with a trade name of Ecoflex[R] F Blend C1200, a biodegradable aliphatic-aromatic copolyester based on the monomers 1,4-butanediol, adipic acid and terephthalic acid in the polymer chain, was supplied by BASF SE, Germany. It had a density of ~1 g/[cm.sup.3], melting range 100-130[degrees]C and melt flow index of 3.2 g/10 min (190[degrees]C, 2.16 kg). The polymer also contained <5% of slip and anti-blocking agents. PLA (procured from Biomer, Germany) with stereochemical purity of >99%, density 1.25 g/[cm.sup.3], [T.sub.g] 50-60[degrees]C, melt flow rate range of 2-4 g/ 10 min (190[degrees]C, 2.16 kg) and melting range 168-172[degrees]C was kindly donated by Prof. Misra at University of Guelph. Canada. Date seeds of Abu Dhabi region were procured locally.

Generation of Bio-Composites

Date seeds were cleaned with 50% [H.sub.2]S[O.sub.4] to remove the surface layers, as described earlier [15], The seeds after cleaning were milled in a ball mill and sieved using 60 [micro]m sieve in the shaker. Prior to melt mixing, the polymers and DSP were dried under vacuum at 70[degrees]C overnight to remove the moisture from the materials. Bio-composites were generated by melt mixing using mini twin conical screw extruder (MiniLab HAAKE Rheomex CTW5, Germany) for three minutes at 80 rpm with batch size of 5 g. A mixing temperature of 145[degrees]C for PBAT and 190[degrees]C for PLA was used. Composites with filler content of 10, 20, 30, and 40 wt% were generated. Disc shaped test samples with 25 mm diameter and 2 mm thickness were immediately prepared by mini injection molding machine (HAAKE MiniJet, Germany) at processing temperatures of 145[degrees]C and 190[degrees]C for PBAT and PLA, respectively. The injection pressure was 700 bar for 6 s, whereas holding pressure was 400 bar for 3 s. The temperature of the mold was kept at 55[degrees]C.

Characterization of Bio-Composites

Rheological behavior of the biopolymers and composites was analyzed using AR 2000 rheometer from TA Instruments. Disc shaped samples were characterized at 140[degrees]C for PBAT and 190[degrees]C for PLA using a gap opening of 1.2 mm. Frequency sweep scans (dynamic testing) of all PBAT composites were recorded at 1% strain from [omega] = 0.1 to 100 rad/s, whereas a strain of 0.3% was used for PLA composites.

For the microscopy analysis, the composite samples were mounted in special holders which at the same time fit in the microtome and were suitable for the examination of the block face by light microscopy and atomic force microscopy. Ultrathin sections (30-90 nm) as well as block faces of the composite samples were obtained using a Leica Ultracut E microtome (Leica, Austria) equipped with a diamond knife (Diatome, Switzerland) at -120[degrees]C. Sections for transmission electron microscopy (TEM) analysis were collected on formvar coated 400 mesh electron microscopy grids, and were examined in a Philips CM 20 (Philips/FEI, Germany) electron microscope at 200 kV at room temperature without staining. TEM image processing was performed using DigitalMicrograph software (Gatan, USA). The block faces of specimens after cryo ultramicrotomy were investigated at ambient conditions using ZEISS Axioplan light microscope (ZEISS, Germany) equipped with ZEISS Axio Cam ICc I CCD camera in reflected polarized light, and by Dimension 3100 AFM/SPM (Veeco, USA) atomic force microscope. The analysis was performed after different periods of time after sectioning (2 h, 24 h, and 30 days). During this time, the samples were kept at controlled temperature (20[degrees]C) under nitrogen atmosphere. AFM images were collected in tapping mode using silicon nitride cantilevers with natural frequencies in the 300 kHz range [force constant 20 N/m, tip radius 10 nm (NT-MDT, Russia)]. AFM image processing was performed using Nanoscope v720 software (Veeco, USA).

The IR-spectra were recorded using a Bruker Equinox 55 FTIR-spectrometer, coupled to a Hyperion 3000 IR-microscope, operated with a single element MCT-detector. All spectra were recorded with 4 [cm.sup.-1] spectral resolution and averaging over 32 scans. The block face surface was contacted without pressing with the diamond window of a Spectratech diamond micro-transmission cell and the IR spectrum of the extracted phase was recorded in transmission mode, using x15 Cassegrain objective. The IR spectrum of the block face was recorded in ATR-mode using x20 ATR-Objective with germanium crystal. Diffuse reflectance infrared spectroscopy study of pure DSP was carried out in HVC microreactor, attached to DRIFT accessory (Praying Mantis) of VERTEX 70 spectrometer, in air atmosphere. Fifty mg of DSP was taken in the reaction chamber and heated at desired temperatures for 10 min before IR acquisition. A heating rate of 10[degrees]C per min was maintained by using an automatic temperature controller. HVC reaction chamber contained a dome with KBr windows and IR acquisitions were done between 4000 and 370 [cm.sup.-1] using OPUS software at 8 [cm.sup.-1] resolution.

Wide angle X-ray diffraction analysis of the composites was performed on Panalytical Powder Diffractometer (X'Pert PRO) using CuK[alpha] radiation ([lambda] = 1.5406 A[degrees]) in reflection mode. Zero-background holder was used to minimize the noise. The samples were step-scanned from 10[degrees] to 70[degrees] 2[theta] at room temperature using a step size of 0.02[degrees] 2[theta] and a step time of 10 s.

Thermal properties of the DSP (vacuum dried), pure polymers (vacuum dried) and bio-composites were analyzed using Netzsch thermogravimetric analyzer (TGA). Nitrogen was used as a carrier gas and the scans were obtained from 50 to 700[degrees]C at a heating rate of 20[degrees]C/min.

LINSEIS STA PT1600 simultaneous thermogravimetric analysis system was also used to measure the weight change during heating in air atmosphere. This system is coupled with a Phipher mass spectrometer (MS) which allowed to determine the elimination of [H.sub.2]O (m/z = 18), C[O.sub.2] (m/z = 44) and other fractions during heating. All measurements were realized with a heating rate of 3[degrees]C/min, using flow rate of 20 ml/min.

Tensile testing of pure polymers and bio-composites was performed on univers+al testing machine (Testometric, UK) using standard ASTM D638. The dumbbell shaped samples with 53 mm length, 4 mm width, and 2 mm thickness were used. A loading rate of 5 mm/min was employed and the tests were carried out at room temperature. WinTest Analysis software was used for the calculation of tensile modulus and other tensile properties. An average of five values is reported.

Biodegradability of biopolymers and composites was analyzed by soil burial test under natural environmental conditions (ASTM D6400). The samples were buried in open pots containing compost soil for gardening. Soil level in the pots was 10 cm, and the samples (with dimensions 15 x 10 x 1.5 mm) were buried 5 cm deep, 3.5 cm apart in longitudinal direction, and 4.5 cm apart in transverse direction. The soil moisture content varied in the range of 60-70%. The soil temperature was recorded in range of 33-37[degrees]C during the day and 20-25[degrees]C during night. The samples were dug out after 30, 60, and 120 days, washed and dried in a vacuum oven at 50[degrees]C followed by surface characterization in light microscope as well as weight loss analysis. The reported weight loss values represented an average of three samples.

RESULTS AND DISCUSSION

In the current study, morphology of composites of biopolyesters with DSP was characterized as a function of time. The processing temperature during composite generation was kept low in order not to degrade DSP, however, was still high enough to ensure good polymer-filler mixing. In an earlier study reporting the reinforcement of bio-polyesters with DSP, the differential scanning calorimetry analysis exhibited changes in the crystallinity of PBAT and PLA because of the addition of DSP [15]. PBAT crystallinity decreased on adding the filler, whereas the PLA exhibited an increase in overall crystallinity as a function of filler content. The peak melting point decreased in PBAT composites, whereas it remained unaltered in PLA based composites. The DSP also had nucleation effect on both the polymers as confirmed by increase in the onset and peak crystallization temperatures with increasing filler content in the composites. Thus, incorporation of DSP to different polymers resulted in different crystalline morphology in the composites. The EDX analysis of the DSP also revealed that the particles had a different composition in the inner core and surface layer. Surface layer contained Si, Ca, and a variety of other elements, whereas soft inner content contained mostly C, O, Si, and small amount of other elements. FT1R spectrum of pure DSP in Fig. la also revealed that the composition of DSP included fatty acids, carbohydrates, proteins, and water [16]. The broad absorption between 3500 and 3000 [cm.sup.-1] is attributed to -O[H.sub.str] vibration of water and -N[H.sub.str] vibration of proteins. The bands present at 1748 and 1250 [cm.sup.-1] are attributed, respectively, to the C = [O.sub.str] and C-[O.sub.str] vibrations of ester carbonyls of fatty acids. The shoulder band between 1650 and 1500 [cm.sup.-1] are corresponding C=O stretching and N-H bending of proteins. Bending vibrations of hydroxyl groups and C-O-[C.sub.str] of carbohydrates could be observed between 1200 and 1000 [cm.sup.-1]. The sharp absorbance around 760 cm 1 is also attributed to C-H out of plane stretching of saturated fatty acids. In addition to these, chemical composition of DSP has also been reported to contain elements like potassium, magnesium, calcium, phosphorous, sodium, and iron [17, 18], The TGA thermogram (in nitrogen) of DSP is shown in Fig. lb. DSP exhibited a weight loss of ~10% in the temperature range of 50-200[degrees]C because of loss of moisture. This moisture loss was retained even after drying the powder overnight at 70[degrees]C under vacuum, which indicated that the moisture was more tightly bound in DSP structure. Figure 1c also demonstrates the TGA thermograms of pure polymers and composites with 10% and 40% DSP content. Degradation peaks around 300[degrees]C corresponding to the degradation of pure DSP were observed in the composites and their magnitude increased corresponding to DSP content. PBAT peak degradation was not affected, whereas PLA degradation was observed to occur at slightly lower temperature. However, the overall thermal stability of the composites was not affected even at 40% DSP content. The moisture loss peak because of DSP observed in Fig. 1b was also absent in the composites confirming that the moisture was lost during the melt mixing process.

Figure 2a and b show the G' and G" of pure polymers and composites as a function of angular frequency. The modulus values in the samples increased with increasing angular frequency indicating resistance of the melt as well as its stability at higher rates of deformation. Enhancing DSP content in the composites also increased the modulus of the composites. In the case of PBAT composites, the G' and G" of pure polymer and 10% composite were similar, followed by a gradual increase in magnitude as the filler content was enhanced further. However, in PLA composites, the enhancement of modulus in composites with 30% and 40% filler was abrupt especially at lower angular frequency values. It indicated that the presence of coarse filler particles and corresponding lower filler-polymer interfacial interactions resulted in higher friction during the rheological testing. At higher frequencies, the curves of PLA composites were observed to merge with each other. Transition point from liquid like to solid like viscoelastic behavior (gel point), at which the polymer acts as true viscoelastic fluid, could also be used to gain insights into the physical nature of the samples. For PBAT composites, G" was always higher than G' indicating that the polymer still retained the dominant viscous behavior even after the inclusion of 40% DSP. In the case of PLA composites, the viscous component dominance was observed for pure polymer and composites till 20% filler content. For composites with higher filler content, the behavior had elastic dominance at lower angular frequency; however, the viscous behavior became dominant at higher values of frequency. The relative tensile modulus of the composites is also demonstrated in Fig. 2c as a function of DSP content. An enhancement of more than 300% in tensile modulus of PBAT bio-polyester was reported at 40% powder fraction. Composites with PLA had marginal enhancement in tensile modulus till 20% filler fraction, beyond which a decrease in modulus was observed because of stress concentration. Extraction of date seed oil during the high temperature melt mixing of polymers with DSP was also suggested [14] which can affect the polymer morphology. However, the mechanical properties of the PBAT and PLA composites did not change with time. It indicated that even in the likelihood of oil generation during melt mixing at high temperature, it did not affect the mechanical performance of the composites. Figure 2d further confirmed the usefulness of DSP as a biofiller for the polymers. The extent of biodegradation of the polymers enhanced as a function of DSP content in the composites. The PBAT and PLA composites with 40% filler content had 11 and 9 times more biodegradation than the pure polymers. respectively. It thus indicated that not only mechanical and rheological properties were improved on DSP addition, it also did not affect the thermal performance of polymers and led to enhanced extent of biodegradation.

The TEM analysis of the samples carried out 2 h and 30 days after sectioning revealed good filler dispersion in the polymer matrices, but had large size distribution. Occasionally, large filler aggregates were observed, especially in the composites with higher filler content. Interestingly, voids around filler particles as well as dispersed in the polymer phase were observed. The section mass-thickness contrast was also observed to decrease during TEM investigation indicating loss of material. Both PBAT and PLA composite systems exhibited the same behavior. These findings were also confirmed by light microscopy analysis of the block face of the composite samples, as shown in Fig. 3. The freshly sectioned block face of the composites with 30% filler content had presence of large voids on the surface. These voids were present even when the block face was analyzed 30 days after sectioning (lower rows in Fig. 3), but the surface features were enhanced. In composites with 10% DSP content, these voids were insignificant in number. Moreover, no significant changes were observed on the block face surface of these composites as a function of time, as shown in the middle rows of Fig. 3. Pure polymers (top rows in Fig. 3) did not show any change in the smooth void-free surface morphology as a function of time. It indicated that the observed phenomenon was related to the DSP and its content in the composites.

Figure 4 a-c show the AFM phase images of PBAT taken 30 days after sectioning and PBAT composite with 10% DSP content 2 h and 30 days after sectioning, respectively. Figure 4d-f show the phase images of same samples at lower magnification. The polymer had distinct phase morphology owing to its crystalline structure and the morphology remained uniform in nature even 30 days after sectioning and no voids or distinct phases could be observed at different magnifications. Thus, it can be concluded that the observed composite morphology in TEM and optical micrographs solely resulted because of the incorporation of DSP in the polymer and the processing conditions used for the composite generation. As also observed earlier in the optical micrographs, the morphology of the composite with 10% DSP did not change significantly over the period of time. A small number of voids (white circular regions in the phase images) were present in both fresh and old samples indicating that the composite maintained dynamic stability with time. In the higher magnification phase image of the aged sample (4C), there was an indication of slight increase in the dark substance. This indicated the generation of another phase in the composite over time, however, its presence was not significant. Similarly, Fig. 5a-c demonstrate the morphology of pure PLA 30 days after sectioning, PLA composite with 10% DSP content 2h and 30 days after sectioning, respectively. The lower magnification images of these samples are depicted in Fig. 5d-f. Similar to PBAT, the pure polymer did not show any voids with tine. The composite with 10% DSP retained its morphology over time thus indicating that the sample was stable. The number of small voids (white circular holes in lower magnification images) in the sample was, however, much higher than the corresponding PBAT composite. Higher compounding temperature used to generate PLA composites would have contributed to this effect. The morphology of the pure PLA was also observed to significantly change on the incorporation of DSP. This has also been confirmed earlier through differential scanning calorimetry studies where crystallinity of PLA enhanced on addition of DSP to PLA [15].

Figure 6a-d show the AFM phase images of the PBAT composite with 30% DSP content. In this case also, the analysis was carried out 2 h (a and c) and 30 days (b and d) after sectioning. The filler aggregates can also be identified in the micrographs. The changes in the morphology in the composites with time were much more significant than the corresponding composite with 10% DSP content. Voids of different sizes were present in the fresh sample, which were retained with time. Also, in the higher magnification phase image of the aged sample (Fig. 6b), the content of dark substance became significantly higher. This phase was observed by the AFM tip to be sticky in nature. Figure 6e-h similarly demonstrate the morphology of the PLA composite with 30% DSP content analyzed 2 h (e and g) and 30 days (f and h) after sectioning, respectively. Similar to PBAT composites, in the composite with 30% DSP content, the morphology changed extensively over the period of time. The large sized voids initially present in the composite were retained in the sample with time, but in addition, significant amount of dark substance or large number of dark "islands" appeared on the surface of the sample (Fig. 6f). These areas were confirmed by AFM tip to be filled with a sticky material and were much softer in morphology. Similar observation was also made around the date seed particles in the seasoned sample and the extent of dark phase was also much higher than the corresponding PBAT composite. One reason resulting in this morphology could be because of the migration of date seed oil, generated during high temperature compounding process, to the surface thus increasing the extent of soft phase. However, a different mechanism was suspected for the generation of observed voids (escape paths) in the matrix. Large amount of bound moisture in the DSP could have resulted in the observed voids. To achieve confirmation of this hypothesis, morphology of the composites with 30% DSP content was analyzed with AFM immediately after composite generation and 24 h after sectioning. Large number of voids was visible in the composites, but there was no indication of the enhanced soft (or dark) phase. This indicated that the suspected migration process of the date seed oil to the sample surface was a slow process and 24 h after composite generation and sectioning were not long enough to allow accumulation of the oil on the surface of the material. It should also be noted that the oil generated from the seed particles was only a small fraction of the total seed powder content and majority of the filler particles were still embedded in the polymer phase. The absence of moisture loss peak in the TGA thermograms of the composites also confirmed the mechanism of void formation as escape of moisture during compounding.

The presence of an oil film on block face surfaces after few weeks of storage made interpretation of the phase images especially challenging. Therefore, AFM images were collected in both regimes: "hard-tapping" and "light-tapping". In this measurement, probes with force constants of around 20 N/m were applied. At hard tapping mode, an [A.sub.sp]/[A.sub.0] ratio (where [A.sub.0] is free air probe oscillation amplitude, and [A.sub.sp] is a set point amplitude) of [less than or equal to]0.1 was used which allows one to apply very high forces during the scanning. This helped to clarify that the residual layer on the sample surface had a liquid nature which was more viscous than the conventional water-based residual layer on the sample surface. The cantilever could easily protrude through it in contrast to the matrix area. At light tapping mode, the [A.sub.sp]/[A.sub.0] ratio was between 5 and 10, which allows one to obtain adequate phase images where harder materials leads to large positive frequency and phase shift (white regions corresponds to the matrix), as compared with the interaction with a soft material (dark region corresponds to the oil islands). Both were measured with respect to the frequency and phase of the freely oscillating cantilever. Therefore, such combination of tuning parameters ([A.sub.sp]/[A.sub.0][less than or equal to]0.1 and 5-10) guaranteed that the phase shift images emphasized fine ultrastructure with superior details, which is barely seen in the height image only.

To further confirm the characteristics of the observed voids and dark substance in the phase images of the composites, height corrugation analysis of composites was carried out. Figure 7a and b show the AFM height and phase images of the PBAT composite with 30% DSP content along with surface topography analysis (surface height or roughness variation) at different places in the micrograph (Fig. 7c). The voids present in the micrograph were observed to have significant height variation confirming their hollow morphology. On the other hand, the phase generated because of the probable migration of oil on the surface showed insignificant height variation confirming that this phase was present on the surface of the sample and did not have any voids. It was further confirmed through the characterization of the dark substance generated by the oil migration in the high magnification images shown in Fig. 7d-e and g-h for PBAT and PLA composites with 30% DSP content, respectively. Significant amount of soft phase was present on the surface. The height variation analysis carried out at two positions on the corresponding height micrographs indicated no variation in the height, thus, re-confirming that the dark phase formed a thin film on the block face surface.

To understand the mechanism of void generation further, TGA-MS studies were carried out on pure DSP, PBAT, and its composite with 30% DSP in air atmosphere as shown in Fig. 8. Similar to Fig. la, the filler was observed to have a weight loss of ~10% in TGA in the temperature range 50-200[degrees]C (Fig. 8a). From the mass loss analysis, this corresponded to only loss of moisture from the powder and no other degradation products were observed. The degradation of DSP occurred at much higher temperature (~300[degrees]C) resulting in the generation of water and carbon dioxide. Thus, as earlier hypothesized, the observed voids in the composites can be related to the escape route of water vapor/steam at high temperature during compounding. This also supported their presence immediately after composite formation as observed earlier. The TGA-MS of pure polymer in Fig. 8b further confirmed that the observed behavior resulted from DSP. as no significant water loss in the compounding temperature region was observed. The degradation pattern for composite sample in Fig. 8c further confirmed that the DSP in composite degraded the same way as pure powder and the generation of date seed oil in the matrix did not lead to any unwanted premature degradation of the system.

The spectroscopic characterization of the hypothesized oil film formed by the surface migration of the date seed oil is demonstrated in Fig. 9 for PLA composite with 30% DSP content. Figure 9a demonstrates the micrograph of the generated oil phase, distributed on the diamond window of a microtransmission cell, when the block face was contacted with it without pressing. Figure 9b compares the IR spectrum of the oil extracted out of the aged block face surface (blue curve) with the spectrum of the block face (red curve). Both spectra showed distinctly different absorption bands. The absorption maxima in the oil spectrum were typical for long-chain aliphatic ester: 2925 and 2855 [cm.sup.-1] (CFF-stretching modes) and 1745 [cm.sup.-1] (C=O-Stretching). The broad absorption band with center at around 1050 [cm.sup.-1] could be caused by the oxidation products, hence it may be referred to as C-O-stretching. Furthermore, the = C-H-stretching mode at 3010 [cm.sup.-1] was hardly detectable in the spectrum of the sample, which was in good agreement with the above mentioned oxidation. The reference spectrum of epoxidized soyabean oil (orange curve) compared well to the spectrum of oil (black curve), thus, clearly showing that the substance on the block face of the sample was an oil. Two spectra of pure vegetable oils (olive and canola oils) were also compared to the spectrum of the oil sample and were observed to have similar absorption bands. The diffuse reflectance infrared spectroscopy study of pure DSP shown in Fig. 9c also confirmed the C=O-stretching band at 1748 [cm.sup.-1]. Its presence in the IR spectrum at 175[degrees]C further confirmed the origin of IR peaks in the oil spectrum observed in Fig. 9b. New peaks corresponding to acid formation were also observed especially at 250[degrees]C indicating initiation of degradation of powder or date seed oil extracted from it.

Figure 10 shows the X-ray diffraction patterns of the pure polymers along with aged composite samples with 10% and 30% DSP contents. The composite samples with 30% DSP content exhibited no specific diffraction peak corresponding to polymer or DSP because of probable masking of these peaks by the oil film accumulated on the surface. These findings confirmed that the migration of the date seed oil generated during the high temperature processing did take place as a function of polymer matrix, DSP content as well as time. It further indicated that the extent of migration can be controlled by suitably selecting composite specifications. Such a phenomenon can also be commercially beneficial as it may lead to the generation of composite materials which have channels for achieving fluid flow and are self-lubricating over a period of time.

CONCLUSIONS

In the current study, properties and morphology of the DSP composites with bio-polyesters along with component migration through the matrix were studied. The PBAT composites exhibited more than 300% increment in tensile modulus with addition of 40% DSP, whereas marginal increment in modulus of PLA was observed till 20% DSP content. Addition of DSP did not affect the thermal stability of the composites. The PBAT composites had dominant viscous response irrespective of the date seed fraction. The PLA composites, on the other hand, exhibited a decrease in viscous behavior as a function of filler content in the matrix. The microscopy as well as TGA-MS analysis of the composites with 30% DSP content confirmed the presence of voids because of the escape of the extensive moisture during the high temperature processing of the composites. The composites also exhibited significant change in the morphology with time. The voids were retained over the period of time, but in addition, a soft and sticky phase owing to the accumulation of date seed oil, generated during high temperature compounding, by surface migration was observed. The accumulation process was also slow in nature and was higher in magnitude in PLA composites probably because of higher compounding temperatures used in the processing. In composites with 10% DSP content, the voids were insignificant in number thus indicating that the extent of filler was responsible for such changes. Analysis of the pure polymers did not show any significant change in the morphology over the period of time thus further confirming the role of the DSP filler in the observed morphological features.

The height analysis of the observed voids as well as dark phase in the composites with higher DSP fraction confirmed large height variation in voids and no height variation in dark phase indicating that the dark phase formed a thin film on the surface. The IR characterization of the extracted oil on the surface confirmed the presence of oil, accumulated by surface migration, which was chemically similar to epoxidized soybean oil. The X-ray diffraction of these composites also revealed masking of the diffraction peaks by the accumulated thin oil film on the surface. As the migration was affected by the polymer matrix and the amount of DSP, controlling these parameters can lead to tunable composite materials with commercial applications. In addition, the oil migration did not affect the mechanical performance of the composites with time and the biodegradation behavior of the composites also enhanced significantly as a function of DSP content. Thus, the usefulness of DSP as functional filler to enhance the polymer properties was successfully demonstrated.

ACKNOWLEDGMENT

The authors are thankful to A.U. Chaudhry at The Petroleum Institute for the rheological analysis.

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Vikas Mittal, (1) Gisha E. Luckachan, (1) Boril Chernev, (2) Nadejda B. Matsko (2)

(1) Chemical Engineering Department, The Petroleum Institute, Abu Dhabi, UAE

(2) Graz Centre for Electron Microscopy, Graz, Austria

Correspondence to: Vikas Mittal; e-mail: vmittal@pi.ac.ae

DOI 10.1002/pen.23955

Published online in Wiley Online Library (wileyonlinelibrary.com).
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Author:Mittal, Vikas; Luckachan, Gisha E.; Chernev, Boril; Matsko, Nadejda B.
Publication:Polymer Engineering and Science
Article Type:Report
Date:Apr 1, 2015
Words:5998
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