Improving the mechanical properties of a pristine system is the main target of developing nanocomposites. The nanocomposites systems were first prepared via intercalation technique with different organophilic montmorillonite (OMMT) loading. Two types of cross-linking techniques were applied, namely, as maleic anhydride polyethylene (MAPE) and electron beam (EB) irradiated system. The effectiveness of these systems was then compared with the control one and analyzed based on the mechanical tests and morphological examination. The mechanical tests revealed that control, MAPE, and EB irradiated systems had attained the optimum mechanical properties at 4 vol% OMMT content. EB irradiated unit of a dose of 100 kGy showed excellent mechanical properties with higher crosslinking degree which were proved by gel content analysis. X-ray diffraction (XRD) analysis confirmed the existence of delamination structure with MAPE and EB irradiation techniques based on the disappearance of characteristic peak. The degree of delamination was further investigated by transmission electron microscope (TEM).
Compatibilizer or cross-linking agent is commonly introduced in a nanocomposite purposely to reduce the surface tension between immiscible polymers and filler as well as to improve the surface adhesion of filler. This is done to obtain the optimum filler dispersion which will improve properties of nanocomposite. The compatibilizer is a functionalized nanocomposite which prevents segregation of the polymer components in the polymer. Moreover, cross-linking agent is proved to enhance the rigidity of the polymer [
Irradiation processing has been used for many years to modify the properties of formed polymer parts. The property enhancements achieved include increased operating temperature, improved mechanical properties, and increased chemical and solvent resistance. It should be noted that polymer structure influences the cross-linking ability of polymer. Chain cross-linking and scission are the two reactions that occur during EB processing of polymers. Polymers typically undergo simultaneous scission and cross-linking, but, in most cases, with one or the other clearly predominating [
In contrast, scission is the opposite process of cross-linking in which the rupturing of C–C bond occurs. Scission reduces cross-linking efficiency and degrades the properties of polymers (chemical resistance, mechanical, and thermal properties). The current study aims to vary and highlight the interest of EB irradiation technique for HDPE/EPDM, where pristine composites and nanocomposites were prepared with different irradiation doses rate and different OMMT loadings. The results obtained for the EB irradiated system were then compared with control and MAPE systems for the determination of the optimum system.
Homopolymer high-density polyethylene (HDPE); (melt index: 3–6 g/10 min, density: 900 kg/cm3) supplied by Cementhai Chemicals Group, Thailand and ethylene propylene diene monomer (EPDM) supplied by Centre West Sdn Bhd, Malaysia were used as the base polymer matrix. MAPE-Polybond@ 3009, obtained from Uniroyal Chemical Company, was used as a coupling agent to improve surface adhesion. Commercially available organophilic montmorillonite (OMMT) surface modified with 15–35 wt% octadecylamine and 0.5 wt% aminopropyltriethoxysilane obtained from Sigma-Aldrich Group, Malaysia was used as reinforcing agent to prepare nanocomposites.
Melt blending of HDPE (70 vol%) and EPDM rubber (30 vol%) as polymer matrix was carried out first in an internal mixer (Thermo Haake Rheomix 600P). Then, HDPE/EPDM blend was mixed with MAPE and OMMT. The content of MAPE agent was fixed at 3 vol% whereas the contents of OMMT were varied between 2, 4, 6, and 8 vol%. The melt mixing conditions were 150°C and at a rotor speed of 100 rpm. Prior to mixing, the polymer matrix and the nano clays were dehumidified in a dry oven at 110°C for a period of 1 hr. Details on the preparation of HDPE/EPDM and HDPE/EPDM filled OMMT are summarized in Tables
Preparation of HDPE/EPDM blend (polymer matrix) via melt blending method.
Polymers | Polymers content (vol%) | Ratio of HDPE to EPDM blend | Total composition |
---|---|---|---|
High density polyethylene (HDPE) | 70 | 70 : 30 | 100 |
Ethylene propylene diene monomer (EPDM) | 30 | 70 : 30 | 100 |
Composition, parameters, and modification of nanocomposites prepared.
Systems | Polymer matrix | Polymer matrix content (vol%) | OMMT content (vol%) | Surface modification | Total composition |
---|---|---|---|---|---|
Control | HDPE/EPDM | 100 | — | — | 100 |
MAPE | HDPE/EPDM | 97 | — | 3 vol% MAPE | 100 |
Control-50 kGy | HDPE/EPDM | 100 | — | Electron beam irradiation at 50 kGy | 100 |
Control-100 kGy | HDPE/EPDM | 100 | — | Electron beam irradiation at 50 kGy | 100 |
Control-150 kGY | HDPE/EPDM | 100 | — | Electron beam irradiation at 50 kGy | 100 |
Control-200 kGy | HDPE/EPDM | 100 | — | Electron beam irradiation at 50 kGy | 100 |
Control/OMMT | HDPE/EPDM | 98, 96, 94, 92 | 2, 4, 6, and 8 | — | 100 |
MAPE/OMMT | HDPE/EPDM | 95, 93, 91, 88 | 2, 4, 6, and 8 | 3 vol% MAPE | 100 |
50 kGy/OMMT | HDPE/EPDM | 98, 96, 94, 92 | 2, 4, 6, and 8 | Electron beam irradiation at 50 kGy | 100 |
100 kGy/OMMT | HDPE/EPDM | 98, 96, 94, 92 | 2, 4, 6, and 8 | Electron beam irradiation at 50 kGy | 100 |
150 kGy/OMMT | HDPE/EPDM | 98, 96, 94, 92 | 2, 4, 6, and 8 | Electron beam irradiation at 50 kGy | 100 |
200 kGy/OMMT | HDPE/EPDM | 98, 96, 94, 92 | 2, 4, 6, and 8 | Electron beam irradiation at 50 kGy | 100 |
Control system = untreated or nonirradiated system; OMMT = organophilic montmorillonite; HDPE = high-density polyethylene; EPDM = ethylene propylene diene monomer; MAPE = maleic anhydride polyethylene.
Subsequently, the blended samples were compression molded as per ASTM-F-412 using a compression molding machine at a temperature range of 135–155°C, 8 tone metric pressures for 14 min.
The melt compounding samples were exposed under high-energy EB irradiation at different unit of dose of 50, 100, 150, and 200 kGy at room temperature using a 3 MeV electron beam accelerator. The acceleration energy, beam current, and dose rate were set to 2 MeV, 2 mA, and 50 kGy/pass, respectively.
Test specimens for analyzing mechanical properties were initially conditioned at
Specimens with dimensions of
Specimens with
Notched Izod impact test as specified by ASTM D256 standard test method was applied by using Ceast 6545/000 model. Specimens with
The gel content of the samples was determined by boiling the samples with xylene for 24 hours in accordance with ASTM D2765 procedure. The extracted samples were vacuum dried to constant weight for 16 hours at 75°C. The gel content was calculated as the ratio of weight of dried sample after extraction to the initial weight of the sample before extraction. The results reported were the average of three specimens. The gel content percentage of cross-linked samples was calculated using the formula below:
X-ray diffractograms of OMMT and the nanocomposites were recorded using Shimadzu 6000 (Japan), X-ray crystallographic unit equipped with nickel filtered Cu K
The morphology of the nanocomposites was observed using a JEOL JEM electron microscope with an accelerating voltage of 100 kV. Ultrathin specimens of 100 nm thickness were cut from the middle section of the compression molded bar using a Leica ultra microtome. The specimens were collected on a trough filled with water and placed on a 200-mesh grid.
The effect of different clay loading on control, MAPE, and EB irradiated systems is demonstrated in Figures
Effect of OMMT content and surface modification on tensile strength for control, MAPE, and EB irradiated systems.
Effect of OMMT content and surface modification on tensile modulus for control, MAPE, and EB irradiated systems.
Tensile strength and modulus of both pristine composite and nanocomposite were further improved with MAPE agent, as evidenced in Figures
An increment of 47.14%, 40.24%, 39.84%, and 35.74% in tensile strength were observed for EB irradiated system with doses rate of 100, 50, 150, and 200 kGy. On the other hand, 49.54%, 31.43%, 23.56%, and 22.33% improvement in modulus were observed at unit of doses of 100, 50, 150, and 200 kGy. The tensile strength and modulus maximized at 18.51 MPa and 701.39 MPa at 4 vol% of OMMT and EB irradiation unit of a dose of 100 kGy. This indicates the formation of radiation induced cross-linking in the rubber and plastic phases as confirmed by the gel content analysis. At higher doses rate, the tensile strength decreased due to the breakdown of the network structure. Evidently, as revealed by XRD and TEM examinations, clay aggregates were broken up and acceptable uniform dispersion of clay particles can be achieved with the aid of EB irradiation.
A moderate increment of the elongation at break with initial incorporation of 2 vol% organoclay loading and followed by a sudden dropping of elongation at break upon 6 vol% loading of organoclay is obtained as in Figure
Effect of OMMT content and surface modification on elongation at break for control, MAPE, and EB irradiated systems.
Further 9.8% reduction in elongation at break at optimum clay loading of 4 vol% was observed for MAPE system. This is believed to be due to the MAPE reduced chain slippage on the surface of the fillers by the reaction both of filler and matrix. As a result, the elongation at break of the MAPE nanocomposite decreased.
EB irradiated system experienced a continuous decrease in elongation at break with increased unit of dose. Reductions of 10.44%, 11.23%, 15.68%, and 21.19% at optimum clay loading of 4 vol% are obtained with unit of dose of 100, 50, 150, and 200 kGy. Generally, increasing radiation dose resulted in reduction of elongation at break of nanocomposite. Therefore, the reduction in elongation at break at higher filler loading could be attributed to the reduced segmental mobility of polymer chains, which resulted from chain degradation. Also, increasing irradiation dose led to increased cross-linked density and hence hindered the extension of chains which resulted in lower value of elongation at break. The enhanced cross-linking density may not necessarily increase the tensile properties of the polymer matrix as in the case of elongation break due to the EB irradiation-induced scission of polymer chain as well as brittleness character.
The effect of organoclay loading on the flexural strength and modulus for control, MAPE, and EB irradiated systems is illustrated in Figures
Effect of OMMT content and surface modification on flexural strength for control, MAPE, and EB irradiated systems.
Effect of OMMT content and surface modification on flexural modulus for control, MAPE, and EB irradiated systems.
A significant increase in flexural strength and modulus were observed with EB irradiated system. An improvement of 39.27% and 39.94% was achieved at 100 kGy unit of dose. As expected, the flexural strength and modulus increased with radiation dose and continued to decrease at doses rate of 150 kGy and above. Such an increase may be attributed to the transfer of electron energy to the polymer. According to Park et al., [
The effect of clay loading on control, MAPE, and EB irradiated system of notched Izod impact strength is depicted in Figure
Effect of OMMT content and surface modification on impact strength for control, MAPE, and EB irradiated systems.
The presence of MAPE significantly modified the impact strength value by 19.43% as compared to control system at the optimum level of 4 vol% OMMT loading. This is believed to be due to the higher diffusion of the lower molecular weight MAPE chains through the polymer matrix and its easier penetration into the octadecylamine group formed at the OMMT surface.
EB irradiated system had obtained the highest value of impact strength as compared to control and MAPE systems. The impact strength of EB irradiated increased with clay loading but decreased as clay loading reached 6 vol% and above. For unfilled composites, an increase of 29.51% is achieved at optimum 100 kGy unit of dose followed by 22.96%, 21.63%, and 20.01% improvement at 50, 150, and 200 kGy. Moreover, the introduction of 4 vol% OMMT loading enhanced the impact strength by 25.34%, 20.22%, 17.54%, and 14.61% at 100, 50, 150, and 200 kGy. This is might be due to the cross-linking effect in pristine composite and nanocomposite systems which resulted in three-dimensional and gel-like structures. In contrast, the reduction in impact strength has contributed to the radiation-induced scission which caused the break age of carbon-carbon bond.
The susceptibility of control composites as well as nanocomposites to cross-linking process was estimated from the gel fraction determination. The gel content of control, MAPE-treated, and irradiated samples increased with the presence of OMMT at optimum loading of 4 vol%. As shown in Figure
Effect of OMMT content and surface modification on gel content.
In this study, the tensile strength property was identified to affect the extent of cross-linking via gel formation. A linear relationship is obtained between the tensile strength and gel formation percentage where higher tensile strength will result in higher gel percentage. An improvement of 26.77% and 30.85%, is achieved for both unfilled composite and nanocomposites with MAPE. This shows that MAPE agent can act as plasticizer and decrease the interfacial energy between the two phases. Thus, the wetting of the OMMT with polymer matrix material has been improved. As a result, the percentage of gel content increased.
For EB irradiated system, the gel content increased rapidly by increasing the radiation unit of dose up to 100 kGy, beyond which it slowly decreased. An enhancement of 62.49%, 70.38%, 53.17%, and 41.81% was observed for pristine composite. Moreover, for nanocomposites system, an improvement of 70.44%, 86.02%, 60.25%, and 50.71% was obtained in gel formation. As EPDM and HDPE are organic polymers which are categorized radiation cross-linkable materials then such formation of cross-links upon irradiation are to be expected in them. Kim and Nho [
Judging from the above results, the optimum OMMT content and EB irradiation unit of dose were taken as 4 vol% and 100 kGy. Therefore, from Section
Interplanar spacing and diffraction angle values for control, MAPE, and EB irradiated systems.
Systems | 2 | Interplanar spacing, |
---|---|---|
Control/4 vol% OMMT | 3.570 | 24.73 |
MAPE/4 vol% OMMT | 3.218 | 28.72 |
EB irradiated-100 kGy/4 vol% OMMT | 2.845 | 31.03 |
As MAPE was added, the (001) peak still appeared, but its intensity was obviously lowered. This suggests that the diffraction peak of MAPE system has shifted toward lower angle and higher diffraction peak which were about 0.36° and 3.9 Å. This shows that the polymer matrix is ready to enter the galleries of OMMT layers resulting in broadening of XRD peak. As a result, the interlayer spacing of the clay increased, and the interaction of the layers should be weakened. However, in the current study, some clay still kept the original stacking condition with MAPE as revealed by TEM micrograph.
On the other hand, nanocomposites with exposure to EB irradiation revealed intercalated structure based on the absence of any basal reflections in the XRD patterns as evidenced by TEM image. It can be seen from Figure
XRD analysis for control, MAPE, and EB irradiated systems.
The nanometer scale dispersion of the treated clays OMMT within the polymer matrix is further corroborated with TEM images as depicted in Figures
TEM micrograph of control system (untreated).
TEM micrograph of MAPE system.
TEM micrograph of EB irradiated system.
Silicate layers with bulky stacks were found to be intercalated in the polymer matrix for control and MAPE systems as evidenced in Figures
However, in the case of EB irradiated system as shown in Figure
The effects of MAPE and EB irradiation as cross-linking agents on the mechanical properties and gel content formation of nanocomposites were investigated in the current study. The findings are then summarized as follows. A good balance of properties in terms of stiffness and strength was achieved at 4 vol% OMMT content. Surface modification through electron beam (EB) radiation has induced high cross-linking as evidenced by gel content analysis, thus enhancing the mechanical properties of unfilled and filled nanocomposites systems. A linear relationship between mechanical properties and gel content has been achieved, where the higher the mechanical properties, the greater the gel formation. High-energy EB irradiation can be an alternative as better impact and surface modification of nanocomposites system in which it can replace the role of chemical cross-linking which has been applied in many studies for decades.
The authors wish to express their gratitude to University Malaysia Perlis, Ministry of Higher Education (MOHE), and International Islamic Malaysia University for the financial assistance and all the staff in MRB, MINT, SIRIM, and IIUM for their help.