INTRODUCTION:

Although synthetic polymers have vast applications in different areas like home appliances, electrical appliances, biomedical devices etc., they lead to environmental pollution due to their non-biodegradability character. Thus, researchers tried to find out a solution to this non-biodegradability. They tried to improve biodegradability by introducing biodegradability onto synthetic polymers by different processes like blending, composite formation, copolymerization and grafting of the synthetic polymers like PP, PVAc, PVC, PE, PS etc. onto a biodegradable natural polymer like starch, gelatin or cellulose to form grafted polymer or composite having improved mechanical properties, biodegradability and renewability. Graft copolymerization was carried out by various researchers [1-8] by using various initiator systems.

Synthetic polymer, Polypropylene (PP) is one of the most widely used vinyl polymer in making electric cables, textiles, household things, as well as in more elaborated materials used in car industry due to its good mechanical properties, chemical stability, high crystallinity, non polarity and lower cost [9].

It is also widely used in preparing medical devices like disposal syringes due to its optimum tensile strength and low level of tissue reaction as compared to other polymers [10]. But due to less impact strength, poor compatibility, non-biodegradability and adhesion towards other materials, a number of modifying approaches have been explored by copolymerization and grafting [11].

Natural polymers have proven to be suitable reinforcement and grafted materials for composites due to good mechanical properties and environmental advantages such as renewability and biodegradability [12]. Biodegradation is a process whereby microbes such as bacteria, fungi, yeasts and their enzymes consume a substance as a food source so that its original form disappears. Under appropriate conditions of moisture, temperature, and oxygen availability, biodegradation is a rapid process. Biodegradation for limited periods is a reasonable target for the complete assimilation and disappearance of an article leaving no toxic or environmentally harmful residue. Gelatin is an animal protein and is a water soluble, biodegradable polymer having large number of industrial, pharmaceutical, and biomedical uses. It can be hydrolyzed by a variety of proteolytic enzymes to yield its constituent amino acids and peptide components. This is a desirable factor in intentional biodegradation [13].

The present study involves the graft copolymerization of synthetic polymer PP onto natural polymer gelatin by involving Glucose-Ce(IV) redox initiator to improve its biodegradability. The biodegradation of the PP-g-gelatin copolymer was monitored as a function of time during soil burial investigation [14]. Graft copolymers have been completely characterized by using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal analysis. Soil burial test was applied to study the biodegradability of grafted PVAc.

EXPERIMENTAL:

Materials

Pure PP granules supplied by Indian Petrochemicals (Vadodara, India) was used for graft copolymerization of gelatin by dissolving it in chlorobenzene (10% w/v), without further purification. Soluble gelatin from Qualigen was used without further purification. Ceric ammonium sulphate, sulphuric acid & glucose supplied by S.D. Fine Chem. Limited, Mumbai, India were used without any further purification. Ceric ion concentration in stock solution was estimated by titrating against standard FeSO₄solution. Water double distillated over alkaline KMnO₄was used for preparation of various solutions. Nitrogen gas obtained from Indian Oxygen Ltd. was made free from oxygen by passing it through Fieser’s solution followed by saturated lead acetate solution and double distilled water.

Procedure of Graft copolymerization

The graft copolymerization of PP onto gelatin was carried out in a three-necked round-bottomed flask, fitted with gas inlet & outlet tubes, under nitrogen atmosphere. 10 % PP solution was prepared by using chlorobenzene as solvent. Requisite amount of PP solution & gelatin were poured into the reaction flask. Then the glucose solution (0.025M-0.05M) & sulphuric acid solution (0.25M-0.5M) were added to reaction vessel followed by addition of conductivity water. The reaction mixture was deaerated by bubbling purified nitrogen through it for half an hour. Then the required amount of Ce-(IV) solution (0.025M-0.05M) was poured into the reaction vessel and was thermostated between 40⁰–60⁰C. The reaction time was varied from 1-4 hours. The reaction mixture was arrested by adding 200 ml of methanol under continuous stirring. The precipitate formed was filtered and washed with hot and cold double distilled water to remove ungrafted gelatin, glucose, and other impurities coprecipitated along with grafted PP. Then it was dried in oven & weighted till a constant weight is obtained. The graft yield (%) was calculated as the percentage increase in weight of dry precipitate over the original weight of PP taken for graft copolymerization [15]. The percentage of grafting was calculated using Fanta’s formula

The percentage of graft yield =

Weight of Polymer in graft	X 100
Weight of ungrafted polymer

Weight of grafted polymer- Weight of ungrafted polymer	x 100
Weight of ungrafted polymer

Characterization:

The grafted samples were characterized by applying different methods like Fourier transform infrared spectroscopy (FTIR), Thermal Analysis, Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) study. Grafted PP samples were analyzed using FTIR using a Thermo Nicolet Avatar 370 FT-IR Spectrometer System. Dry air was used as the chamber purge stream for all samples. The scanning resolution was set to 1 nm with a total of 1024 scans per sample. The FTIR spectra were obtained at room temperature over a spectral frequency range of 500-4000 cm^-1using Omnic software.

The thermal analysis like TGA, DTG and DSC were carried out simultaneously by using a PYRIS Diamond TG/DTA thermal analyzer. The sample was kept in alumina pan, the reference material was alumina powder and thermal study was carried out at heating rate 10°C/min under 200 ml/min flow rate of nitrogen atmosphere. The measurements were run from room temperature to 500°C.

Micrographs of grafted polymer samples were studied with JOEL-JSM-5800 Scanning Electron Microscope (Japan). The specimens were coated with 50 nm thick gold films in an automatic sputter coater (Polaron) to avoid surface charging under electron beam.

Powder X-ray diffraction of gelatin–grafted PP samples were studied by using Bruker AXS D8 Advance (Germany) using Nickel filtered CuKα radiation and Copper as target at wavelength of 1.54 Å. Goniometer speed was kept at 2°/min. Wide angle X-ray scattering (WAXS) pattern of the samples were obtained using the DIFFRAC Plus XRD Commander software and analysis was done by DIFFRAC Plus (Version 8.0) software. The range of scanning angle for the sample was kept in the range 2θ=10–120°.

Preparation of film by casting method

15% solutions of PP and PP-g-gelatin samples were prepared using chlorobenzene as solvent. The solutions were stirred vigorously with magnetic stirrer for 1 hr to get homogeneous mixture of samples. Then the solutions were poured onto clear and dry plates to cast films of different sample solution. The glass plates were shaken back and forth to spread the solution uniformly, then they were dried for about an hour in an oven maintained within the temperature range of 30-45° C. Then the weights of the samples were recorded.

Biodegradability study

Pure PP and PP-g-gelatin films of various graft yields were taken to test the biodegradability of the films. The biodegradability test was carried out by soil burial test [16].

Soil burial test

The soil for soil burial test was obtained from a fertile land. The soil was made free from large clumps, plant debris etc. and was kept in several earthen pots. Thin film samples of uniform thickness (about 0.5mm) of known weight were buried in these pots at a depth of about 10cm. The soil containing the samples was watered daily. The samples were removed periodically after a fixed time intervals, washed thoroughly with water, and then dried in a vacuum oven for 24 hr within the temperature range of 40-45° C. The weight of the films were recorded and used to calculate the extent of biodegradation of the samples. The percentage of biodegradation was calculated by using the formula as given below:

Percentage of biodegradation = × 100

Fig. 1: Variation of graft yield (%) with reaction time

Fig. 2: Variation of graft yield (%) with reaction temperature

RESULTS AND DISCUSSION:

The effect of various parameters such as reaction time, reaction temperature, concentration of initiator, PP, gelatin, sulphuric acid and glucose on grafting were studied to optimize the reaction conditions for maximizing graft copolymerization. The effects of various parameters were studied by varying one parameter and keeping all other parameters constant.

Effect of reaction time on graft yield

The variation of graft yield (%) with reaction time is shown in Fig. 1. It was found that the graft yield (%) increases with increasing time from 1 to 3 hr and with further increase in time period, it decreases. The initial increase in graft yield (%) with increasing time period is attributed to the fact that with increasing reaction time, the concentration of initiating species increases and with further increasing in time period, destruction of initiating species occurs and thereby leading to decreasing graft yield (%) time period [6, 17].

Fig. 3: Variation of graft yield (%) with concentration of Ce(IV)

Fig. 4: Variation of graft yield (%) with concentration glucose

Effect of reaction temperature on graft yield

The effect of reaction temperature on graft yield (%) [Fig. 2] was studied within the temperature range 40-60°C. From the Fig. 2, it is evident that with increase in temperature from 40°C to 45°C, the graft yield (%) has increased but on further increasing the temperature the graft yield was found to decrease. This can be explained by assuming that on increasing temperature from 40°C - 45°C, solubility of gelatin, swellability of PP and decomposition of initiator increases leading to the formation of more free radicals and generation of active sites on the polymeric backbone and absorption of gelatin on the PP backbone favors grafting. But above 45°C, the interaction of PP macro radicals with Ce (IV) ions destroy the activity of the initiating species and thus leading to premature termination of the growing polymeric chain. Further, there may be occurrence of chain transfer which thus leads to decreased graft yield (%) at higher temperatures [17].

Fig. 5: Variation of graft yield (%) with concentration of H₂SO₄

Fig. 6: Variation of graft yield (%) with amount of gelatin

Effect of concentration of Ce(IV) on graft yield

Fig. 3 shows the variation of concentration of Ce(IV) with graft yield (%). From the figure it is observed that with increasing concentration of Ce(IV) from 0.025-0.05 M the percentage of graft yield increases. A possible explanation for such observation may be that an intermediate complex of Ce (IV) - PP might be formed at the beginning which dissociated to give PP macroradicals. The macroradicals thus formed react with gelatin to form graft copolymer. By increasing the concentration of Ce(IV) ion, there was an increase in the concentration of active sites of PP and gelatin, hence the graft yield was increased on increasing the concentration of Ce(IV) ions in the solution [14].

Effect of concentration of glucose on graft yield

The effect of glucose concentration on graft yield (%) is shown in Fig. 4. From the figure, it was observed that with the increase in concentrations of glucose from 0.025M-0.05M, the percentage of graft yield decreases due to formation of radical scavenger and the increase in the rate of termination of growing chains [5].

Fig. 7: Effect of amount of PP on graft yield (%)

Effect of concentration of sulphuric acid on graft yield

The role of sulphuric acid on grafting of gelatin onto PP can be drawn from the results shown in the Fig. 5. It is observed that with increase in concentration of acid from 0.25-0.5 M, the graft yield has increased. This is attributed to the fact that at higher concentration of sulphuric acid the swellability of polypropylene has increased, which made gelatin more accessible to the active sites on PP for grafting in comparison with low concentration of sulphuric acid [6].

Effect of concentration of gelatin on graft yield

The effect of concentration of gelatin on graft yield (%) is given in Fig. 6. It was observed that on increasing the amount of gelatin from 1 to 1.5 g the percentage of grafting increases and with further increase in amount of gelatin, graft yield (%) decreases. The increasing trend may be due to increasing concentration of gelatin macro free radicals at higher concentration of gelatin and the rate of their combination with PP has become faster. The decreasing trend at further higher concentration of gelatin may be attributed to the fact that the concentration of gelatin macro radicals increases further with increase in concentration of gelatin and the rate of their termination are faster than the rate of their combination with PP molecules [18]. Further, the rate of diffusion of gelatin onto surface of PP decreases with further increase in concentration of gelatin which hinder the further grafting of PP.

Effect of concentration of PP on graft yield

From the variation plot of concentration of PP with graft yield shown in Fig 7, it is observed that on increasing the concentration of PP the percentage of graft yield has decreased which is due to increase in viscosity of the reaction mixture which hindered the rate of initiation and chain propagation.

Table 1 Characteristic FTIR bands of PP and PP-g-gelatin

Wavelength (cm^-1)	Assignment	Ref.
Aliphatic Hydrocarbons
~2920	-CH₃ asymmetric stretching of PP	[14]
~2956	- CH₂ asymmetric stretching of PP	[14]
~1166	- CHCH₃ group of PP	[14]
~998	Helix chain of PP	[14]
2922,2850	C-H stretching in gelatin	[18], [19]
Aliphatic Ethers
1167-1034	-C-O-C– ether linkage between PP and gelatin in PP-g-gelatin	[14]
Aliphatic amide
~3450, ~3423	-NH stretching of secondary amide in gelatin	[18], [19]
~1550-1500	-NH bending in gelatin	[18], [19]
~ 1680-1640	>C=O stretching in gelatin	[18], [19]
~670	-NH out-of-plane wagging in gelatin	[18], [19]
~1647	>C=O of amide group in PP-g-gelatin	[14]
~3400-3200	-NH₂ group of PP-g-gelatin	[14]

Characterization of the grafted polymer

FTIR Spectroscopy

The FTIR spectra of gelatin, pure PP and PP-g-gelatin are shown in Fig. 8 (a), (b) and (c), respectively. The FTIR spectra of gelatin shown peaks at 3450 cm⁻¹ and 3423 cm⁻¹ due to -NH stretching of secondary amide, C=O stretching at 1680 cm^–1 and 1640 cm^–1, -NH bending between 1550 cm^–1 and 1500 cm^–1, -NH out-of-plane wagging at 670 cm^–1, and C-H stretching at 2922 cm^–1 and 2850 cm⁻¹. It can be seen from Fig. 8 (b), that absorption bands at 2920 cm⁻¹ is mainly for the presence of -CH₃ asymmetric stretching of PP, 2956 cm^-1 for -CH₂ asymmetric stretching of PP, 1166 cm⁻¹ for –CHCH₃ group and 998 cm⁻¹ for helix chain PP. The bands around 1030 cm⁻¹ (C-O-C stretching) present in the FTIR spectrum of gelatin are attributed to its saccharide structure [18, 19]. From the Fig. 8 (c) it is found that new peaks are observed at 1647 cm⁻¹ due to >C=O of amide group, 1167-1034 cm⁻¹ due to -C-O-C– ether linkage between PP and gelatin and around 3400-3200 cm⁻¹ due to –NH₂ group of PP-g-gelatin [17]. The FTIR bands for all the possible functional groups are discussed in the Table 1.

Table 2 Percentage of Biodegradation of PP and gelatin grafted PP

Sample No.	Graft yield (%)	Biodegradation (%)
Sample No.	Graft yield (%)	30 days	60 days	90 days	120 days	150 days
I	0	0	0	0	0	0
II	38.42	10.38	15.59	22.49	26.62	32.64
III	49.91	14.87	21.38	28.81	35.15	41.35
IV	66.36	20.23	29.37	38.39	46.52	56.11
V	83.86	22.86	52.26	63.22	77.41	83.21

Thermal analysis

TGA, DTG and DSC thermograms of PP, gelatin, PP-g-gelatin samples are represented in the Figs. 9, 10 and 11, respectively. The study of thermal behavior has been carried out to understand the thermal changes that might occur and to correlate the effect of grafting on thermal transitions. The TGA curve for gelatin shows a three stage thermogram, whereas, PP and grafted PP samples do not exhibit three stage thermogram. It has also been observed that gelatin is stable upto 250ºC, PP is stable upto 400ºC and grafted PP samples are stable upto 310ºC. Further, it has also been observed that complete degradation of gelatin occurs around 700ºC, whereas, PP and grafted PP samples degraded completely around 500ºC. The thermal stability of grafted samples are intermediate between PP and gelatin. They are less stable than PP but more stable than gelatin. This is in accordance to the fact that crystalline structures are thermally degraded at higher temperatures as compared to amorphous structures [20]. This explains the grafted samples are more crystalline than gelatin which can give toughness to the film formed from grafted samples but still then imparting to biodegradability. The crystallinity can also be observed from the X-ray diffractograms.

From the DTG curves given in Fig. 10, it is clear that the decomposition temperature, T_d, of gelatin is around 300ºC, whereas for PP and grafted PP samples, T_dis around 450ºC. It can be observed from the DSC thermogram shown in Fig. 11 that the area under melting endotherm, ΔH_m, for the grafted samples, decreases with increase in percentage of grafting, which indicates that grafted PP is less stable in comparison to virgin PP.

X-Ray Diffraction analysis

The powder wide angle X-ray diffraction (XRD) pattern of PP, gelatin and PP-g-gelatin samples are shown in Fig. 12. It can be seen from the figure that PP, gelatin, and PP-g-gelatin samples shows crystalline peak within 2θ value 10-50°. X-ray diffractograms of gelatin shows that there is no crystalline peak which clears the fact that it is mostly amorphous in nature. But pure PP shows crystalline peaks and as well as amorphous region. Thus, PP is semicrystalline in nature. However, grafted PP samples [Fig. 12] have shown there is decrease in percent crystallinity with increase in graft yield percent from 38.42 to 83.86. This can be attributed to the formation of crosslinks by gelatin molecules on the backbone of PP [21]. It is also visualized that, in addition to the decrease in percent crystallinity of PP, there might be trapped amorphous gelatin between the PP chains, which also has contributed towards the decrease in percent crystallinity of the grafted samples. The decrease in crystallinity of PP has been due to the disruption of crystallites and the dilution of inherent crystallinity of PP [22]. The percentage of crystallinity is determined by using the formula as given below:

Where, A_c - area of crystalline phase, A_a -area of amorphous phase, and X_c- %age of crystallinity.

Scanning Electron Microscope Study

SEM micrographs [Fig. 13(a–d)] have shown that virgin gelatin is neither uniform in size nor uniform in shape. However, SEM micrographs for PP have shown almost uniform spherical shapes having rough surface structures. And the grafted samples have shown neither planar nor uniform surface but they contain almost uniform spherical shapes. It has been observed that with the increase in percentage of grafting, some ungrafted gelatin have remained associated with the grafted sample. On the whole, SEM micrographs have provided substantial morphological evidences in favor of grafting of PP onto gelatin [15].

Biodegradability Study

The data given in Table 2 have clearly demonstrated that grafting of PP onto gelatin has induced biodegradability in PP; otherwise PP has shown no weight loss within the studied time intervals of 150 days. The data given in Table 2 have also indicated that the extent of biodegradation of PP-g-gelatin has increased on increasing the percent graft yield.

CONCLUSION:

The extent of graft yield (%) could be controlled by controlling the concentration of initiator, sulfuric acid, glucose, PP, gelatin, and also by reaction time and temperature. By means of chemical method of graft copolymerization, polypropylene has been successfully crosslinked with gelatin. Grafting is evident from the FTIR and XRD study. In case of PP-g-gelatin, the spectrum shows new peaks are observed at 1647 cm⁻¹ due to >C=O of amide group, 1167-1034 cm⁻¹ due to -C-O-C– ether linkage between PP and gelatin and around 3400-3200 cm⁻¹ due to –NH₂ group of PP-g-gelatin. The grafted samples are more crystalline than gelatin which can give toughness to the film formed from grafted samples but still then imparting to biodegradability due to the presence of gelatin in PP moieties. The decomposition temperature, T_d, of gelatin is around 300ºC, whereas for PP and grafted PP samples, T_dis around 450ºC. The DSC thermograms shown that the area under melting endotherm, ΔH_m, for the grafted samples, decreases with increase in percentage of grafting, which indicates that grafted PP is less stable in comparison to virgin PP. Morphological studies of grafted samples by SEM have given evidence in favor of surface grafting of PP onto gelatin. Grafting of biodegradable gelatin moieties on synthetic polymers has also induced biodegradability, and the percentage of biodegradation has increased with the increase in percentage graft yields of PP onto gelatin.

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Received on 14.11.2011 Modified on 12.12.2011

Asian J. Research Chem. 5(2): February 2012; Page 205-214