A series of low-density polyethylene (LDPE)/ethylene propylene diene terpolymer (EPDM) blends with various compositions have been prepared by melt mixing followed by injection molding. These specimens are irradiated at 40, 80, 120 and 160 kGy electron beam radiation. The gel content increases with increase in EPDM and as well as EB dose. Storage modulus (E’) and loss modulus (E”) are decreased with increase in EPDM content. Storage modulus continues to increase and loss modulus keeps on decreasing with radiation dose. Interestingly, damping property is found to be more for EPDM rich blends, which again decreases upon irradiation. Morphology of fractured surface of LDPE/EPDM shows that with increase in EPDM content, the size and depth of the cavity becomes larger and deeper indicating higher ductility. But, EB crosslinking makes the surface smoother and the smoothness keeps on increasing with increase in dose rendering stiffness to the samples.
Blending of a thermoplastic with an elastomer leading to a thermoplastic elastomer has become a practice for last forty years. Due to their advantages in processing of thermoplastic, with elastomer of excellent physical pro- perties, it is gaining wide varieties of applications. This is a widely used technique nowadays in industries. There are various thermoplastics, which are unique for easy processing, availability, and low cost. Among the polyolefins, low density polyethylene (LDPE) and linear low density poly ethylene (LLDPE) are widely used plastics in the packaging and consumer industries because of their advantages like higher tensile strength, stress crack resistance, flexibility, thermal, chemical and excellent dielectric properties [
Mechanical, thermal, rheological and morphological properties of PE/EPDM blends have been investigated and reported [
LDPE (MFI 4 gm/10 min, density of 0.922 gm/cc) in form of pellets has been supplied by Reliance Petrochemicals. EPDM (pellet form, Mooney viscosity, ML1+4 at 125˚C 20, NORDEL IP 4520, ethylene content = 50% with 4.9% of ENB, MFI 10 gm/10 min and density of 0.88 gm/cc) is supplied by Dow chemical. These are used to prepare blends. LDPE is blended with EPDM in different compositions using twin screw extruder with the temperature profile of 120˚C: 140˚C: 160˚C: 180˚C at 80 rpm without any crosslinker. Both components are mixed in various weight proportions to prepare blends (100/0, 70/30, 50/50, 30/70, 0/100). For study purpose dumble shaped samples are prepared by injection molding at 180˚C. The codes of the samples are provided in
Samples | 1 | 2 | 3 | 4 | 5 |
---|---|---|---|---|---|
LDPE | 100 | 70 | 50 | 30 | 0 |
EPDM | 0 | 30 | 50 | 70 | 100 |
Sample code | L 100 | LE73 | LE55 | LE37 | E 100 |
“L” stands for LDPE and “E” stands for EPDM.
are irradiated by high energy electron beam in an inert environment using 2 MeV, 20 kW electron beam accelerator (Model ILU-6) under forced air cooling at a radiation dose of 40, 80, 120 and 160 kGy (kilo Grey). Only one side of the sample is exposed to irradiation, as the thickness of the sheet is 2 mm, which is thin enough for penetration of the electron beam of 2 MeV energy. The distance of the sample from the scan horn is 20 cm and the conveyer speed is set at 0.94 m/min. The dose rate is 10 kGy/pass and beam current is 1 mA.
Gel fractions are measured by solvent extraction technique using xylene as solvent. The samples are extracted in hot Xylene for 48 hrs at 110˚C. Extracted samples are dried in a vacuum oven at 80˚C till constant weight. The gel content (% gel fraction) is determined using the following formula:
The FTIR spectra of both unirradiated and irradiated samples (2 mm thick) have been recorded on a Bruker- Alpha’s Platinum ATR model. Samples are characterized on Attenuated total reflection (ATR) mode in wave number ranging from 500 - 4000 cm−1. Dynamic mechanical thermal analysis (or Dynamic mechanical analysis) is carried out for LDPE-EPDM blends to determine the viscoelastic properties of the samples. The test is performed in tension mode using EPLEXOR 150 N (Gabo Qualimeter) instrument. The temperature range is −150 to 80˚C with a heating rate 3˚C/min. The dimension of test samples is 20 mm × 6 mm × 2 mm. The Analysis is investigated on temperature sweep mode at constant frequency of 1 Hz to find out storage modulus (E’), loss modulus (E”) and loss tangent (tanδ) for unirradiated and irradiated blends. The samples of LDPE/EPDM blends fractured in liquid nitrogen are characterized by SEM (scanning electron microscopy) using JEOL, JSM-5400 model. To increase the conductivity, fractured surfaces are gold sputtered before investigation under the scanning electron microscope.
The susceptibility of LDPE, EPDM and various LDPE/EPDM blends, towards radiation induced cross-linking process, in absence of crosslinker, is estimated from the gel fraction determination. Results are presented in
proves up to 120 kGy, in both the polymers, crosslinking process dominates over the chain scission process. Similar increasing trend is noticed even at 160 kGy for LDPE. This is due to it’s higher crystallinity and lower speed of crosslinking. Interestingly, the gel content of EPDM is decreased at 160 kGy. At this higher dose EPDM is degraded. From the figures it is again obvious that, at a certain dose, EPDM shows higher gel fraction than LDPE and that is true for all studied doses. That is due to higher crosslinking ability of EPDM than LDPE. The crosslinking ability of EPDM is more due to higher amorphousness and molecular structure of EPDM [
So, it is expected that with increase in EPDM content in blends, gel fraction would keep on increasing. That is the finding throughout the experiments, which has been discussed later. Interestingly, as in LDPE and EPDM, the gel contents of LDPE/EPDM blends are also in increasing trend with increase in irradiation dose up to 120 kGy. But at 160 kGy, all LDPE/EPDM blends degrade showing reduced gel fractions. Chain scission is initiated in blends due to presence of EPDM component. In LE 73 blends the percentage of gel fraction are observed to be 60.34%, 74.11%, 80.63% and 79.21% for different electron beam doses (40, 80, 120 and 160 kGy). In LE55 blends, the gel fractions are higher than that of LE 73 blends due to greater amount of EPDM content in blends. The increased gel content in LE 55 blends are 63.39%, 77.00%, 85.02% and 77.15% at 40, 80, 120 and 160 kGy. It is again seen that LE 37 blend shows higher degree of change in gel fraction compared to LE 73 and LE 55 blends. The values of gel fractions for LE37 blends are 67.29%, 80.04%, 86.51% and 75.56 % at the above mentioned doses.
FTIR spectra of LDPE, EPDM and various unirradiated and irradiated LDPE/EPDM blends are taken and represented in
plenty of C-C bonds, so expectedly no new peak corresponding to −C-C- bond formation will appear rather the intensity corresponding to various modes of C-C bond vibration will alter, producing signal of radiation crosslinking. However, new bonds corresponding to chain scission may appear. A prominent strong peak appears at 719 cm−1 (
Absorbance at 675 cm−1 and 1647 cm−1, which are due to stretching of −C=C- bond, are increased after irradiation [
The dynamic mechanical properties (storage modulus, loss modulus and tanδ) of pure LDPE, EPDM and LDPE/EPDM blends before irradiation are reported in Figures 4(a)-(c). To understand the change quantitively storage modulus, loss modulus as well as transition temperatures are tabulated in
The storage moduli of all blends are decreased with temperature following similar fashion as LDPE and EPDM due to increase in flexibility. Again, it is interesting to note that the modulus of neat EPDM is significantly lower than that of neat LDPE in the whole range of temperature studied. This is due to the greater stiffness originated from compact organized crystalline structure of LDPE. The blends, on the other hand, exhibit storage moduli in between the two neat polymers in the whole temperature range.
The alterations of dynamic mechanical properties of neat polymers and blends, with radiation dose and temperature are illustrated in Figures 5(a)-(c), Figures 6(a)-(c) and Figures 7(a)-(c) for LE73, LE55 and LE37 respectively.
Upon irradiation storage modulus of LE73 increases, which continues to increase with dose up to 120 kGy (
Samples | E’ (−140˚C) (MPa) | E’ (−75˚C) (MPa) | E’ (+25˚C) (MPa) | E” (−140˚C) (MPa) | E” (−75˚C) (MPa) | E” (+25˚C) (MPa) | Temp. corresponding to tanδmax (LDPE) | Temp. corresponding to tanδmax (EPDM) |
---|---|---|---|---|---|---|---|---|
L100 | 3780 | 2120 | 327 | 374 | 150 | 54 | −132, −15 | - |
E100 | 1720 | 926 | 9 | 227 | 73 | 2 | - | −30 |
LE 73 | 2570 | 1610 | 197 | 339 | 129 | 55 | −130 | |
LE 73 40 kGy | 3130 | 1820 | 240 | 330 | 115 | 31 | −15.4 −132 | |
LE 73 80 kGy | 3320 | 1910 | 215 | 315 | 109 | 24 | −15.2 −131 | |
LE 73 120 kGy | 3600 | 2030 | 340 | 290 | 94 | 22 | −15.2 −134 −14.9 | |
LE 55 | 2350 | 1345 | 62 | 314 | 109 | 11 | −128 | −23.6 |
LE 55 40 kGy | 2460 | 1552 | 141 | 282 | 96 | 10 | −127.8 | −33.2 |
LE 55 80 kGy | 2530 | 1665 | 176 | 290 | 92 | 8 | −129.7 | −29.9 |
LE 55 120 kGy | 2860 | 1820 | 224 | 262 | 78 | 13 | −127.3 | −32.1 |
LE 37 | 2096 | 1196 | 18 | 294 | 104 | 9 | −129 | −25 |
LE 37 40 kGy | 2230 | 1230 | 69 | 255 | 97 | 7 | −128 | −22.5 |
LE 37 80 kGy | 2425 | 1293 | 108 | 259 | 84 | 5 | −129 | −22.3 |
LE 37 120 kGy | 2700 | 1480 | 180 | 234 | 76 | 7 | −129 | −22.1 |
radiation, degree of crosslinking depending on irradiation dose and blend composition. This three dimensional interchain crosslinked network makes the materials stiffer and rigid, which provide greater resistance to dynamic deformation. That’s why irradiation leads to increase in storage modulus in the whole range of temperature studied degree depending on irradiation doses. All crosslinked samples show reduced storage moduli with temperature due to same reason as unirradiated, i.e. increase in chain mobility at high temperature.
Loss modulus measures the energy dissipated as heat, representing the viscous portion.
Neat LDPE shows multiple loss peaks in temperature range −140˚C to 80˚C, from 374 MPa, the loss modulus reduces to 150 MPa at −75˚C, which again reduces to 54MPa at 25˚C (
It is seen that at −140˚C loss modulus (E”) of blends (LE73, LE55 and LE37) reduces to with increase of radiation doses (40 to 120 kGy,
From
In LE55, a prominent peak, corresponding to Tg of EPDM occurs at −23.6˚C. For LE 37, that peak appears at −25˚C but with less peak height and peak area than those of pure EPDM. From these observations it reveals that LDPE/EPDM blends are partially compatible as the Tg of EPDM is increasing depending on LDPE content [
On crosslinking all three blends LE73, LE55 and LE 37 show almost similar behavior in changing damping property with irradiation dose and temperature (
For crosslinked LE73 transition occurs around −130˚C and −15˚C like uncrosslinked one. Both are due to LDPE. Second transition merges with the glass transition of EPDM (−30˚C). Due to higher quantity of LDPE in blends β transition dominates in LE73 blends masking the glass transition of EPDM. After crosslinking there is no change of transition temperatures (
γ transition (−130˚C, Tgs of LDPE) in LE55 and LE 37 does not change much after crosslinking, though in blends there is no sharp peak of LDPE before and after crosslinking.
In LE 55, the transition at −23.6˚C (ascribed to Tg of EPDM), shifts towards lower temperature on crosslinking at 40, 80 and 120 kGy. This may be due to higher intraphase crosslinking than interphase crosslinking in blend, which reduces the compatibility. The second peak of LDPE (−15˚C) is merged with this peak. In LE 37, second transition occurs at −25˚C, which remains almost unchanged after crosslinking at all three doses.
Morphologies of fractured surfaces of unirradiated and irradiated LE 73, LE 55 and LE 37 blends are investigated by SEM and pictures are provided in Figures 8(a)-(f). In LE 73 blend, the fractured cracks and fissures can be seen across the coarse surface where small domains are dispersed in matrix indicating less ductile surface of blend (
In LE 55 untreated, both components are in same proportions and the fractured surface looks like honeycomb
(
However, these organized structures were destructed and surface becomes smoother with disappearance of roughness after irradiation with different doses. The size of cavities continues to reduce with radiation doses. It is seen from the Figures 8 (c)-(f) that surface becomes smoother with increase in irradiation dose.
At 40 kGy, the size and depth of cavities of LE55 get reduced and large number of flow patches are generated across the surface and surface starts to become smoother (
From the increase in percentage gel of LDPE and EPDM and their blends with increased radiation dose, it is seen that the crosslinking efficiency of EPDM is higher than LDPE. The trend of change of storage modulus with blend composition before and after irradiation is similar (higher the EPDM content less is the modulus), only the values are higher for irradiated samples that keeps on increasing with dose. On irradiation loss modulus continues to decrease with dose for a particular blend. For EPDM rich blend the damping properties is found higher though at lower temperature range LDPE rich blend shows greater damping properties due to synergism of glass transition of LDPE and vibration of amorphous EPDM chains. Upon irradiation damping properties keep on decreasing with dose. EPDM rich blends show more ductile surface with higher matrix yielding, supporting less storage modulus. But irradiation makes surface smooth and smoothness keeps on increasing with irradiation dose revealing increased compatibility (higher stiffness) between two phases through crosslinking.
Bhuwanesh KumarSharma,Subhendu RayChowdhury,Prakash AnnaMahanwar,Kuppa Siva SankaraSarma, (2015) Structure-Property Relationship in Terms of Dynamic Mechanical Properties of High Energy Radiation Treated Industrially Important Thermoplastic Elastomer Blend. Advances in Materials Physics and Chemistry,05,383-398. doi: 10.4236/ampc.2015.59039