Grafting of maleic anhydride and amine derivative onto natural rubber for high performance elastomeric applications.
Under an increasing awareness of fuel shortage in future , natural rubber becomes an important choice due to its renewable resource and excellent physical properties. Synthetic rubbers are generally made from nonrenewable oil-based resource . However, NR contains unsaturated molecules of cis-1,4-polyisoprene  which has some drawbacks; such as poor oil and heat resistance due to its non-polar nature. Therefore, the application of NR is limited. The modified NR is preferably used to prepare the products with better properties than those of unmodified one. Modification of NR with various forms have been investigated in order to improve the properties such as epoxidized natural rubber (ENR) [4-5], halogenated natural rubber (HNR)  graft copolymers of NR with vinyl monomers, i.e., styrene , methyl methacrylate (MMA)  and natural rubber grafted with phosphate functional groups i.e., dimethyl (methacryloyloxymethyl)-phosphonate (NR-g-PDMMMP) . Therefore, it could be widely extended the uses of NR in industries. The rise in glass transition temperature by grafting reaction causes a reduction in elastic properties of these rubbers . Furthermore, the preparation of modified natural rubber from latex is complicated. Because it is necessary to do some treatments to get modified natural rubber. To overcome these problems, modification of NRs using melt blending at high temperature has been used to prepare modified NR to avoid complicated grafting methods together with higher thermal stability, oil resistance and elastic properties.
In this work, an attempt has been made to prepare modified forms of NR, i.e. maleated natural rubber (MNR) and natural rubber grafted with amine derivative (NR-g-HPM). Melt blending method using an internal mixer at high temperature was employed to prepare the samples. Mooney viscosities, Mooney relaxation and oil resistance properties of NRs were investigated. Thermal stability of NR was also determined by thermogravimetric analysis (TGA) and dynamic mechanical thermal analysis (DMTA). The properties of unmodified and modified natural rubber samples were studied for comparison.
Materials and Methods
Air dried sheet (ADS) of natural rubber was used as rubber chain in graft copolymerization, i.e. MNR and NR-g-HPM. It was supplied by Khuan Pun Tae Farmer co-operation, Phattalung, Thailand. MNR and NR-g-HPM were prepared by melt blending of ADS with maleic anhydride (Fluka chemical, Switzerland) and N-(4-Hydroxy phenyl) maleimide at 145[degrees]C and 200[degrees]C, respectively.
3.2.1. Preparation of functionalized natural rubber:
184.108.40.206. Maleated natural rubber (MNR):
MNR was first prepared by melt blending NR and maleic anhydride (MA) in the internal mixer at 145[degrees]C for 10 min with a rotor speed of 60 rpm under normal atmosphere. The condition and formulation used to prepare MNR is as shown in Table 1. MNR was purified by re-precipitation technique for FTIR analysis.
220.127.116.11. Natural rubber grafted with N-(4-Hydroxy phenyl) maleimide (NR-g-HPM):
--Synthesis of N-(4-Hydroxy phenyl) Maleimide (HPM):
The HPM was prepared by gradually adding paminophenol (24.0 g) into a solution of maleic anhydride (21.0 g) in dimethyl formamide (DMF) (25 ml). Polyphosphoric acid (PPA) was added and the mixture was stirred for 2 h at 80[degrees]C. The reaction mixture was then cooled down and poured into cold distillated water. The orange precipitate was separated and washed several times with distillated water. The HPM product was recrystallized with isopropanol, filtered and vacuum dried for 2 h at 40[degrees]C .
--Preparation of NR-g-HPM:
The NR was first masticated for 2 min using an internal mixer at 195[degrees]C at a rotor speed of 60 rpm. 8 phr of HPM was then added into the mixing chamber. The mixing was continued for 10 min. The amount of the chemicals used is shown in Table 2. The resulting product was purified and then characterized by FT-IR.
3.2.2. Characterization of functionalized natural rubber:
After purification, the resulting functionalized natural rubber samples were analyzed by using FTIR. The infrared spectra were recorded using the Omnic ESP Magna-IR 560 spectrophotometer (Perkin-Elmer, USA), in a range of 4000-400 [cm.sup.-1].
3.2.3. Properties of functionalized natural rubber: -Mooney viscosity:
The Mooney viscosity of unmodified and modified natural rubber samples were measured using Mooney viscometer, model: Visc TECH+, (TechPro Inc, Cuyagoya Fall, USA). The test was performed at 100[degrees]C using a large rotor with a preheating time for 1 min and testing time of 4 min, (i.e., ML (1+4), 100[degrees]C).
Mooney relaxations of the modified NRs were tested according to ASTM D 1646-03 using Mooney viscometer (Visc TECH) with relaxation mode. At the end of the test within 0.1seconds the rotation of the disk reset from the zero torque point to the static zero for a stationary rotor, and record the torque at minimum rates. The relaxation data was recorded for 4 min starting typically 1 second after the rotor was stopped.
Thermal analysis was performed using TGA and DMTA techniques. TGA with DTG was carried out using a Perkin Elmer TGA Pyrist (Perkin Elmer Co., Ltd., Massachusetts, USA). The samples (10 mg) were degraded under a nitrogen flow in thermo balance at a heating rate of 20[degrees]C /min.
DMTA was performed using Perkin Elmer DMTA V, (Perkin Elmer Co. Ltd, Massachusetts, USA). The experiment was conducted in a dual cantilever bending mode at frequency of 1 Hz and strain magnitude of 0.1% with a heating rate of 10[degrees]C/min over the range of temperature of -100 to 100[degrees]C. The dimensions of the samples used were ~2 m thick, 10 mm length, and 20 mm width.
Swelling test was performed at 23 [+ or -] 2[degrees]C for 70 h. In this work, ASTM oil number 1 was used as a test liquid. The degree of swelling was calculated as below:
Change in mass (%) = [[W.sub.S] - [W.sub.0]]/[W.sub.0] x 100
where [W.sub.0] and [W.sub.S] are the mass of the specimen before and after immersion in the test liquid, respectively.
Result and discussion
4.1. FTIR analysis of NR with different forms:
Figure 1 shows FTIR spectra of MNR and NRg-HPM compared with the unmodified NR. Several new absorption peaks at different wave numbers were observed in the spectra of modified NRs, as shown in Table 3. Absorption peak at wave number 835 [cm.sup.-1] corresponds to =C-H out of plane bending is observed for all the three types of natural rubber samples. An absorption peak at wave number 1784 [cm.sup.-1] is appeared for MNR, this is attributed to the bending frequency of polymer anhydride present in MNR. In case of NR-g-HPM, a peak is observed at 3483 [cm.sup.-1] corresponds to the bending frequency of hydroxyl maleamide in NR-g-HPM. These results are in good agreement with various data available in the literature [11-13]. This suggests that natural rubber molecules are grafted with anhydride and hydroxyl maleamide groups. It confirms the grafting reaction occurred by the formation of MNR and NRg-HPM.
4.2. Mooney Viscosity:
Figure 2 and Table 4 show Mooney viscosity of MNR and NR-g-HPM prepared using melt blending techniques, and it is compared with unmodified NR. It is seen that Mooney viscosity values of modified NR samples i.e., MNR and NR-g-HPM are lower than that of the unmodified NR. This is a consequence of the destruction of MNR and NR-g-HPM chains due to shearing and heating during the preparation. Therefore, MNR and NR-g-HPM molecule contain low molecular weight molecules resulting in short chain entanglement and hence a lower shearing torque is required to deform the material during Mooney test.
Figure 3 shows the plot of log Mooney unit from the stress relaxation test of NR for different forms. Mooney relaxation was also tested using Mooney viscometer with relaxation mode. The relaxation exhibits the elasticity of the materials. That is, the plot with higher slope shows higher elasticity. The figure indicates a remarkable effect of functional group on the elasticity of natural rubber. It can be seen that NR without modification has the lower slope compared to MNR. It shows the higher elasticity of MNR compared to unmodified NR. This may be attributed to the higher interaction between the polar functional groups of its molecules or self crosslinking under heat treatment , as shown in Scheme 1. The plot for NR-g-HPM shows the lowest slope or lowest elasticity. The mobility restriction of HPM pendent group on NR-g-HPM chain leads to a reduction in elasticity. Therefore, the elasticity in descending order as follows: MNR > NR without modification > NR-g-HPM.
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4.3. Thermal Characterization:
One of the most important applications of thermogravimetry is to study the thermal stability of polymer products. Thermogravimetric curves provide information about the decomposition mechanism for various materials. Figure 4 shows the thermogravimetric curves of unmodified natural rubber, MNR and NR-g-HPM samples. Unmodified NR is compared with MNR and NR-g-HPM modified natural rubbers. The degradation temperature (Td) for various forms of natural rubber is depicted in Table 4. Unmodified natural rubber shows the lowest decomposition temperature of 354.1[degrees]C. The modified forms of natural rubber degraded at higher temperatures at 355.9 and 358.7[degrees]C for MNR and NR-g-HPM, respectively. An increase in decomposition temperature is observed for the modified natural rubber samples. NR-g-HPM exhibits the higher decomposition temperature. It shows the improved thermal stability of natural rubber by grafting with HPM. This is due to the phenol group present in HPM functional group. It acts as an antioxidant. Therefore, it is capable of scavenging and destroying chain propagation especially in the form of peroxy, alkoxy and hydroxy radicals by donating active hydrogen atoms. The radicals are generally converted to inert byproduct, such as alcohol and water molecules. They do not abstract more hydrogen atoms from rubber molecules to create new active free radicals . Hence, it is seen that NR-g-HPM provides the highest thermal stability among the series of various forms of natural rubber studied.
Figure 5 shows the variation of tan 5 with temperature based on DMTA characterization. It can be seen that MNR shows the lowest glass transition temperature ([T.sub.g]) at -51.7[degrees]C, while natural rubber without modification and NR-g-HPM show [T.sub.g] at 49.8 and -48.2[degrees]C, respectively. This is also attributed by the highest chain flexibility of MNR due to the interaction between functional groups or the formation of self cross-links during heat treatment, as shown in Scheme 1. However, NR-g-HPM provides higher [T.sub.g] than that of NR without modification. This is attributed to the lower chain flexibility in case of NR-g-HPM because of the mobility restriction of rubber chain by HPM pendent group. These results show good agreement with elasticity from Mooney relaxation test. MNR shows the highest elasticity and NR-g-HPM shows the lowest elasticity.
4.4. Oil resistance:
One of the important characteristics of elastomeric materials is their ability to transport molecules through the solid. The sorption or swelling of small molecules in rubber depends on a number of factors such as crosslink density, degree of unsaturation, presence of fillers, temperature, etc. The swelling experiment has been conducted in oil. Figure 6 shows the degree of swelling for unmodified natural rubber, MNR and NR-g-HPM. It is seen that the degree of swelling for both modified natural rubber samples exhibits lower values compared to unmodified NR. This can be ascribed in terms of chemical interaction between polar functional groups in the molecules. This reason is responsible for the higher oil resistance in case of both modified natural rubbers. The chemical interaction between polar functional groups does not allow the chains to rearrange easily under oil stress. Therefore, modified natural rubber samples show higher oil resistance compared to unmodified natural rubber.
Modified natural rubber samples in the form of MNR and NR-g-HPM were successfully prepared by melt blending technique at high temperature. FT-IR spectra have been used to confirm the grafting of functional group onto natural rubber backbone. An improvement in thermal stability and oil resistance was observed for modified natural rubber samples. This may be due to the presence of polar functional groups in its molecule. Among the three different types of natural rubbers, MNR exhibited the highest elastic and oil resistance properties, while the NR-g-HPM provides the highest thermal stability. In summary, the modified forms of natural rubber exhibited promising overall properties particularly oil and heat resistance. Therefore these modified forms of natural rubber can be used to blend with other polar polymers to design the most desirable thermoplastic elastomers for a specific end use requirement
Received: 28 February 2014; Revised: 25 May 2014; Accepted: 6 June 2014; Available online: 20 June 2014
The authors gratefully acknowledge financial support by the faculty of science research fund, Prince of Songkla University, Thailand. Also, we would like to thank department of material science and technology for all facilities.
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(1) Ekwipoo Kalkornsurapranee, (1) Worasak Phetwarotai, (2) Jobish Johns
(1) Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat-Yai, Thailand, 90112.
(2) Department of Physics, Rajarajeswari College of Engineering, Bangalore-74, India.
Corresponding Author: Ekwipoo Kalkornsurapranee, Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat-Yai, Thailand.
Tel: +668-41986578, E-mail: firstname.lastname@example.org
Table 1: Formulation used for grafting of MA on to NR. Ingredients phr Natural rubber 100 Mastication for 2 min at 145[degrees]C Maleic anhydride (MA) 8 Grafting state of 10 min at 145[degrees]C Table 2: Formulation used for grafting of NR and HPM. Ingredients phr Natural rubber 100 Mastication of 2 min at 195[degrees]C N-(4-hydroxy phenyl) maleimide (HPM) 8 Grafting state of 10 min at 195[degrees]C Table 3: Various absorption peaks for NR, MNR and NR-g-HPM. Wavenumbers Functionals NR types ([cm.sup.-1]) Unmodified MNR NR-g- NR HPM 835 =C-H out of / / / plane bending of NR 1275 -C-H stretching / / / of NR 1523 -C=C -- -- / stretching, aromatic ring 1664 -C=C stretching / / / of NR 1710, 1732,1736 -C=O -- / -- stretching, carbonyl group 1784 -C=O stretching -- / -- of polymeric anhydride 1854 -C=O stretching -- / -- of succinic anhydride 2900 -C-H stretching / / / of NR 3483 -O-H stretching -- -- / of hydroxyl maleamide Table 4: Mooney viscosities, Mooney relaxation, degradation temperatures ([T.sub.d]), glass transition temperatures ([T.sub.g]), oil swelling (%) of unmodified NR, MNR and NR-g-HPM NR Forms Unmodified NR MNR NR-g-HPM Mooney 87.7 76.9 58.2 viscosity (ML1+4, 100[degrees]C), MU Slope from 0.10 0.12 0.07 the plot of Mooney relaxation Td, [degrees]C 354.1 355.9 358.7 Tg, [degrees]C -49.8 -51.7 -48.2 Oil swelling, % 53.7 [+ 44.6 [+ 51.6 [+ or -] 0.33 or -] 0.62 or -] 0.95
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|Title Annotation:||Research Article|
|Author:||Kalkornsurapranee, Ekwipoo; Phetwarotai, Worasak; Johns, Jobish|
|Publication:||American-Eurasian Journal of Sustainable Agriculture|
|Date:||Jun 20, 2014|
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