Synthesis, Characterization and Biodegradable Studies of Oil Based Polymers from Triethyleneglycoldimethacrylate and Vinylacetate
S. Begila David* and G. Allen Gnana Raj
Department of Chemistry, Scott Christian College (Autonomous), Nagercoil, Kanyakumari District, Tamil Nadu, 629 003, India.
*Corresponding Author E-mail: begila@rediffmail.com
ABSTRACT:
An attempt to find alternative ways of producing polymers from non-petroleum oil specifically, soybean oil, was carried out. The idea originated because of the need to produce polymers from renewable oil sources as well as with cost effective materials. The acrylated epoxy resin was obtained by insitu epoxidation of soybean oil under controlled reaction conditions. Two new polymeric materials were prepared by varying the concentration of the comonomers triethyleneglycoldimethacrylate (TEGMA), and Vinylacetate (VA). The new polymeric materials exhibited tensile stress-strain behaviour ranging from soft rubbers to relatively brittle plastics. They were characterized with respect to their resistance to chemical reagents and mechanical properties such as tensile strength, percentage elongation and hardness. Thermal characterization such as differential thermal analysis (DTA) and thermo gravimetric analysis (TGA) were carried out. Biodegradability test was carried out using hydrolytic test, chemical resistance test and soil burial test. In soil burial degradation, the polymer sample showed severe surface degradation by the attack of microorganism. These soybean oil based polymers possessed mechanical properties comparable to those of commercially available rubbery materials and conventional plastics and this may serve as replacements in many applications.
KEYWORDS: biodegradable polymers, comonomer, brittle, crosslinking, thermal analysis
INTRODUCTION:
During the last decade of the 20th century world economics had an impact on the increasing interest that was occurring in the scientific community regarding the development of polymers from non-petroleum based oil. Scientific research in this area focused on using vegetable oils as the oil base. More attention is being paid to studying and developing environment biodegradable plastics in order to retard or eradicate plastic pollution1,2. Current interest in cheap biodegradable polymeric materials has recently encouraged the development of such materials from readily available, renewable, inexpensive natural sources such as starch, polysaccharides and edible oils3. Among the triglyceride oils, soybean attracts great interest because of low cost and biodegradability4-7. Soybean oil contains 85% unsaturated oleic, linoleic and linolenic fatty acids. This high degree of unsaturation makes it possible to polymerize it into useful materials. Epoxidation and ring opening reaction with haloacids or alcohols, ozonolysis and hydration are some of the common methods for functionalization of unsaturated vegetable oils8-12.
Several researchers have attempted to use epoxidized soybean oil for polymeric and composite applications13-15. However due to its low reactivity and its tendency to intramolecular bonding epoxidized soybean oil normally has a low cross linking density and therefore limited thermal and mechanical properties. Such transformation make the triglyceride capable of reaction via ring opening or polycondensation polymerization. These particular chemical pathways are also accessible via natural epoxy and hydroxyl functional triglycerides16-19. Reaction of the epoxy-functional triglyceride with acrylic acid incorporates acrylates into the triglycerides20. These monomers can then be blended with a reactive comonomer and cured by free radical polymerization. Acrylated epoxidized soybean oil renders thermosetting compounds by thermal polymerization or copolymerization21,22. A major advantage of thermoset composites over thermoplastic composites is the low viscosity of the resin facilitating the impregnation of the fibres23. The cellular structure of foamed plastics imparts them a lower density, lower heat conductivity, superior sound absorption and in some cases better resilience than ordinary plastics24. Thermosets are thus often processed at room temperature, while thermoplastics need melting. Thus, the processability of thermosets is more versatile and a wide range of process techniques are available. Furthermore, various new materials ranging from soft elastomers through ductile to relatively brittle plastics can be tailored by varying the stoichiometry, the type of the oil, and the crosslinking agent. These materials possess viable mechanical properties and thus are suitable replacements for petroleum-based polymeric materials in numerous applications.
The present work reports the conversion of soybean oil to useful solid polymers. We thus report our progress in understanding the synthesis, mechanical, thermal and chemical properties of soybean oil based biodegradable thermosetting polymers. The advantages of these polymeric materials are their low cost, availability from a renewable natural resource and their possible biodegradability. Today, natural oils and fats are considered to be the most important class of renewable sources for the production of biodegradable polymers.
MATERIAL AND METHODS:
The soybean oil in this study were regular food grade oil, purchased from supermarket and was used without further purification. Hydrogen peroxide (AR Rankhem) was used for epoxidation. Epoxy resin was then acrylated using acrylic acid (E. Merck India) and triethylamine (E. Merck India) Triethyleneglycoldimethacrylate (TEGMA), vinyl acetate (VA) purchased from Sigma Alrich were used as comonomers. Benzoyl peroxide is used as the initiator and dimethyl aniline as the accelerator.
Epoxidation of soybean oil using 30% hydrogen peroxide was carried out by peracetic acid method 25. Soybean oil was mixed with glacial acetic acid and 49% sulphuric acid. Hydrogen peroxide was added drop by drop for about 2 hours. The reaction mixture was refluxed with constant stirring for about 10 hours at 70-80oC. The epoxised resin was separated from the unreacted glacial acetic acid by washing with warm water. Epoxidised resin was then acrylated using acrylic acid and triethylamine was used as catalyst 26. Benzene was added as solvent and the reaction mixture was refluxed for about 3 h at 80-100oC .
The tensile tests were conducted according to ASTM D638M using an Instron Universal testing machine at a crosshead speed of 100 mm/min. The dumbell shaped test specimen had a guage section with a length of 50mm, a width of 10 mm and a thickness of 3 mm. The tear strength of the polymer was determined using a Zwick Universal testing machine as per ASTM Standard D624. Indentation hardness (Shore A) was determined as per ASTM Standard D2240. Hardness tester durometer was used. The polymers were subjected to differential thermal analysis (DTA), thermogravimetric analysis (TGA). Acid, alkali and solvent resistance were estimated according to ASTM standard D3137,C267. Polymeric samples (3 x 1x 0.1cm) were immersed in the medium (100ml) for a total duration of 60 days under ambient conditions. The medium was changed and fresh medium was added at an interval of one week. The loss of weight was determined using an electronic weighing balance.
Biodegradability test of the polymer sheet was carried out using soil burial test. Polymer film (15 x 15mm and thickness 0.2mm) were buried in the soil in which the relative humidity maintained was 50-60% by spraying water. The soil was conditioned for four weeks before it is used for the actual test. After 28 days, the buried polymer films were dug out at regular interval of 7 days from the soil and were washed with deionized water. Finally the specimens were dried in vacuum at 30oC to constant weight. Scanning Electron Microscope (SEM) was used for assessing surface damages of polymeric sheet subjected to soil burial test.
RESULTS AND DISCUSSION:
Polymer preparation:
Acrylated epoxy resin [SAE] was then copolymerised with Triethyleneglycoldimethacrylate [TEGMA], and vinyl acetate [VA]. Benzoyl peroxide is used as the initiator and dimethyl aniline as the accelerator. The mixture is heated in the oven for 1 h at 100oC yield thermosetting polymers. Two different polymeric materials were prepared by varying the concentration of comonomers, TEGMA and VA. The nomenclature adopted in this work for the samples are SV1 and SV2. SV1 series of samples corresponds to a polymer prepared from acrylated epoxy resin (SAE) and vinylacetate at different concentrations. SV2 series of samples corresponds to the polymer prepared from SAE, vinylacetate and TEGMA at different concentrations. The samples prepared have been designated as follows. As an example SV1SAE50VA50 means that the sample contains about 50% by weight of acrylated epoxidized resin of soybean oil (SAE) and 50% by weight of vinylacetate.
|
Polymer |
Tensile Strength MPa |
Tear strength KN/m |
%Elongation at break |
Hardness Shore(A) |
Crosslink density Mol/m3 |
Molecular Weight g/mol |
|
SV1SAE50VA50 |
Brittle |
Brittle |
Brittle |
62 |
7.53x103 |
132.80 |
|
SV1SAE75VA25 |
Brittle |
Brittle |
Brittle |
66 |
7.55x103 |
132.45 |
|
SV1SAE25VA75 |
Brittle |
Brittle |
Brittle |
63 |
7.56x103 |
132.28 |
|
SV2 SAE50VA 40 TEGMA 10 |
2.4±0.4 |
10.06 |
6.6±3 |
36 |
4.29x103 |
233.10 |
|
SV2SAE 60VA 25 TEGMA 15 |
2.7±0.3 |
10.02 |
6.3±4 |
34 |
4.30x103 |
232.56 |
|
SV2SAE40VA40 TEGMA 20 |
2.6±0.2 |
10.10 |
6.8±2 |
31 |
4.32x103 |
231.48 |
Mechanical properties:
Mechanical properties of thermosetting polymers such as tensile strength, tear strength, elongation at break (%), shore hardness are given in table 1. The polymers in this study SV1 and SV2 series of samples are thermosets due to cross linking through the multiple carbon-carbon double bonds present in the acrylated epoxy resin of soybean oil and the comonomers TEGMA, and VA 27.
The two samples of varying composition obtained using different comonomers varies from rubber to hard plastics. They appear to be plastics with crosslink densities ranging from 7.56 x 103 to 4.29 x 103 . The results indicate that the products are typical thermosetting polymers with densely crosslinked structures. Crosslink densities differ significantly from one another, which is a direct result of the different degrees of unsaturation of different comonomers employed. SV1 samples of varying composition possess high crosslink density and SV2 samples have the lowest crosslink density. Crosslinking increases the tightness of the polymer network and reduces the molecular mobility of the chains between the junctions28. The magnitude of the rubbery modulus indicates that these soybean oil polymers have crosslink densities than those of polyester thermosets29-31. As a result the number of conformation that a polymer chain can adopt decreases, as the crosslink densities increases. Thus crosslinking increases the stiffness of the polymers. As a crack is initiated during the tensile process, more energy is required for the crack to propagate in a material with a higher degree of crosslinking until failure occurs. Thus crosslinking also increases the ultimate tensile strength of the polymers.
Brittle materials have the low toughness, while ductile materials are very tough. In a tensile test, the tensile strength and elongation at break both contribute to the toughness of a material. The addition of comonomer vinylacetate to TEGMA introduces some degree of flexibility in the rigid polymer. This is attributed to lower molecular weight between the crosslinks and higher crosslink density of the ductile polymer. In the present work, the rigid brittle polymer possesses higher shore hardness when compared to that of the ductile polymer. Increase in elongation in ductile polymer is also due to the flexibility of the chain introduced by the comonomers.
Thermal Properties:
All the samples were subjected to thermogravimetric analysis. The samples of the same series exhibited common thermal characteristics and hence thermogram of samples from each series are presented here. TGA and DTA thermograms are given in fig 1(a) & 1(b). The bulk polymer is thermally stable under 100o C followed by three decomposition temperature regions 100-350oC (stage 1) 350-470oC (stage 11) and above 470oC (stage111). The first decomposition step (stage 1) of the bulk polymer is mainly due to evaporation and decomposition of the free oil. The decomposition temperature of the highly crosslinked polymer is above 400oc at the heating rate of 20oC/min, approximately that of the decomposition of the bulk polymer (stage11). This step corresponds to degradation and char formation of the crosslinking polymer network. Above 530oC the char residues gradually oxidize in air. Therefore the last temperature region (stage 111) above 470oC is due to oxidation of the char residues. This temperature region are almost the same for the two polymers.
Fig 1(a). TGA/DTA curve of SV1
Fig 1(b). TGA/DTA curve of SV2
TGA and DTA data for soybean oil polymers is given in table 2. In order to compare the relative thermal stability the temperature for weight losses at 5%, 50%, 90% is presented in table 2. The 5% weight loss is considered to represent the beginning of mass loss. It is observed that 5% weight loss is above 150oC in all the polymers. This indicates that no thermal degradation takes place during synthesis of these polymers . From the value of the temperature of 90% weight loss it is observed that SV1 is more stable than SV2.
Table 2 : TGA/DTA data for Soybean oil polymers
Polymer |
Exotherm |
Temperature (oC) of weight loss |
|||
|
First |
Second |
5% |
50% |
90% |
|
|
SV1SAE50VA50 |
350 |
530 |
180 |
400 |
490 |
|
SV2SAE50VA40TEGMA10 |
400 |
550 |
195 |
440 |
460 |
The DTA thermogram in SV1 sample show an endothermic peak at 100oC which corresponds to hydrates but this endothermic peak is absent in SV2 samples. But it does not show any endothermic peak for softening and also do not exhibit endothermic peak distinctly for chain scission and thermal decomposition. But two exotherms invariably seen in SV1 and SV2 samples are given in table 2. The first exotherm at 350oC in SV1 and 400oC in SV2 samples are relatively weak corresponds to the cleavage of the long alkyl side chain. The second exotherm at 530oC in SV1 and at 550oC in SV2 samples are strong and corresponds to the decomposition and char formation of crosslinking polymer network.
Biodegradation Studies:
Hydrolytic Studies:
Weight loss % in hydrolytic stability is given in table 3. Hydrolytic stability test was carried out according to ASTM D3137. Weight loss of polymer samples in media like water, ethanol and brine solution (1N Nacl) was determined. The degree of biodegradation was estimated from the weight loss analysis of samples 32.
Weight loss % = {(Wo-W1)/Wo} x 100
Wo- Weight of the original polymer
W1- Weight of the residual film after degradation at each designated days. Hydrolytic degradation was a slow process and negligible weight loss was observed for short interval.
|
Sample |
Weight loss% |
||
|
Water |
Ethanol |
Salt solutions |
|
|
SV1 SAE 50 VA50 SV1 SAE 75VA25 SV1 SAE 25VA75 SV2 SAE 50VA 40 TEGMA 10 SV2 SAE 60VA 25 TEGMA 15 SV2 SAE 40 VA40 TEGMA 20 |
0.01 0.01 0.02 0.03
0.04
0.04
|
0 0 0 0
0
0
|
0.01 0.02 0.02 0.05
0.04
0.04
|
Sample |
Weight loss% |
||
|
Acid |
Base |
Oxidant 30% H2O2 |
|
|
SV1 SAE 50 VA50 SV1 SAE 75VA25 SV1 SAE 25VA75 SV2 SAE 50VA 40TEGMA 10 SV2 SAE 60VA 25 TEGMA 15 SV2 SAE 40 VA40 TEGMA 20 |
0.98 0.97 0.98 2.24 2.13 2.20 |
12.61 12.43 12.72 15.16 15.48 15.37 |
3.42 3.23 3.31 4.81 4.93 4.96 |
Chemical resistance test:
Chemical resistance test was carried out according to ASTM D267. Degradation of polymer in dil.HCl (1N), NaOH (1N) and 30% H2O2 was studied and weight loss was estimated. Weight loss % are given in table 4.
In chemical resistance test the weight loss of all samples was faster at high pH greater than 10. It is because base promotes hydrolysis by providing the strong nucleophilic reagent OH-. The degradation was faster with increase in pH 33.
After chemical resistance test film surface becomes irregular and large number of pits, granular formation, cracks appeared. The number of granular formation, cracks and pits becomes deeper with the increasing exposure time. Especially after increasing the exposure time, the cracks were more pronounced and fragmentation of film occurred.
Soil burial degradation test:
The soil burial degradation test of polymer were conducted as per ISO 846:199734. In soil burial degradation, the effect of microorganisms arises on the surface of the polymer film 35,36. The calculation of biodegradation rate from weight loss of polymer films in soil burial test constitutes a practical problem, since the soil sticks onto the film surface, and weight measurements are not accurate. Kimura et al. in his study of degradation of plastics in the soil emphasized that the degradation of plastics was mainly caused by bacteria and fungi and that different soil conditions affected the rate of degradation of plastics37. Fig.2 shows the SEM micrograph of polymeric samples before and after soil burial test.
Fig.2(a) SV1SAE50VA50 Fig.2(c) SV1(a)SAE50VA50
Fig.2(b)SV2SAE50VA40 Fig.2(d) SV2(a)SAE50VA40
TEG10 TEG10
Fig 2(a) and 2(b) shows the SEM micrographs of SV1 and SV2 samples before soil burial test Fig 2(c) and 2(d). The original polymer film exhibits a relative smooth surface. However, at 28 days in the natural soil, large number of holes, cavities, pinhole were observed in the polymer film, indicating that the polymer surface was attacked by the microorganism under soil environment35,37. SV2SAE50VA40TEGMA10 sample is more affected than the other samples.
CONCLUSION:
The mechanical properties of new polymeric materials exhibited tensile stress-strain behaviour from soft rubbery materials through ductile to relatively brittle plastics depending upon the comonomers used. SV2 polymeric samples with different composition possessed higher mechanical properties than other polymers. When the amount of resin was equivalent to the amount of comonomers, the resulting polymers possessed the highest toughness. Generally, the ultimate tensile strength increased and the elongation at break decreased when increasing the degree of crosslinking.
Thermogravimetric analysis shows that there are three decomposition region which correspond to the evaporation and decomposition of the unreacted free oil present in the bulk polymer, degradation and char formation of the crosslinked polymer and finally due to the oxidation of the char residues in air. SV1 polymer samples was observed to be thermally more stable than other polymer samples.
Degradation study shows that the degree of biodegradation and percentage weight loss was higher in chemical resistance test than hydrolytic stability test. The degradation increases with increase in time. In soil burial degradation, the morphology of the polymer film changes upon degradation for all samples. Stains were also observed which are suspected to evidence that polymer samples are degraded by attack of microorganism in the soil. The new materials have mechanical properties that are comparable to those of petroleum-based polymer materials and thus can serve as replacements in numerous applications.
ACKNOWLEDGEMENT:
This work was financially supported by the University Grants Commission (UGC), New Delhi.
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Received on 13.04.2011 Modified on 02.05.2011
Accepted on 13.05.2011 © AJRC All right reserved
Asian J. Research Chem. 4(7): July, 2011; Page 1092-1096