Exploring the effect of electron beam on swelling,gel fraction,mechanical and thermal properties of ethylene acrylic elastomer/millable polyurethane rubber blends

Abstract

Ethylene acrylic elastomers (AEM) and millable polyurethane (MPU) blend have been prepared successfully using dicumyl peroxide (DCP) system and the blend ratio is optimised on the basis of the tensile results. The optimised unvulcanised blend ratio (60:40) is subjected to different electron beam (EB) dose to obtain uniform three dimensional crosslinking networks. The characteristic shifting of = CO vibration in the Fourier-Transform Infrared spectra confirms the formation of hydrogen bonding between the two phases. The phase morphology of the two phases in the optimised blend has been captured in SEM and the results show the successful integration of MPU in AEM phase. It is found that the swelling of the irradiated blend decreases with increase in EB dose, whereas the increase in gel fraction and the crosslink density is due to the irradiation-induced crosslinking. The thermal stability of the irradiated blend has been improved in compression to the pristine form of individual phase.

GRAPHICAL ABSTRACT

Keywords

Functional materials adhesion elastic properties ethylene acrylic elastomers millable polyurethane electron beam interface

Introduction

Polymer blends target to blend together different polymers or elastomers for completing each other's compatibility properties. Polymer blends, particularly the elastomers/thermoplastic blends which are known as thermoplastic elastomers become very important material due to their unique properties, which bridge the gap between conventional elastomers and thermoplastics [1].The thermoplastic elastomers exhibit characteristics of base elastomers with the processing characteristics of thermoplastics due to their soft rubbery phase and hard thermoplastic phase. The finely divided elastomeric phase in the thermoplastic component of the blend is the main cause for exhibiting superior properties in comparison to the individual component of the blend [2, 3].Wang et al. fabricated the blend of thermoplastic polyurethane (TPU) and ethylene propylene diene elastomers (EPDM) and the formation of EPDM network in TPU matrix was reported at the loading of 8 wt-% of EPDM to TPU matrix [4].

Usually, the characteristic properties of elastomers and thermoplastic elastomers are enhanced by incorporating different curing agents such as sulphur, peroxide, metal oxides etc, [5]. However, these chemical based crosslinking phenomena have certain disadvantages such as the formation of toxic byproducts and it requires a high temperature, which leads to the thermal degradation of a thermally sensitive component present in the polymer. The problems arising in chemical crosslinking systems can be resolved by using radiation-induced crosslinking phenomena. The faster curing, environmental friendly and uniform crosslinking at room temperature are the main advantages of radiation crosslinking over the conventional crosslinking systems. At room temperature the crosslinking in the elastomeric systems can be induced by irradiation process using electron beam (EB), UV and γ radiation sources depending on the desired properties and applications. Exposure of elastomeric phase to electromagnetic radiation (EB, UV, γ, etc.) causes crosslinking, chain scission and modification of network structures which signifies the final product is the combination of the above three phases of reactions.

Among the electromagnetic radiations, EB irradiation has better acceptability in industries because of its simplified operational technique [6, 7] with very low waste materials. EB crosslinked elastomeric materials exhibit a high degree of crosslinking, excellent mechanical properties with very high fluid resistance properties [8]. Giri et al., explored the influence of the effect EB on the dynamic mechanical properties and thermal stability of LLDPE/PDMS (Lineal low density polyethylene/Polydimethylsiloxane) blend and the enhancement of storage modulus and improvement in thermal stability up to a certain dose of EB dose is observed [9]. Mostly the EB crosslinked elastomer blends exhibit excellent oil and heat resistant properties of high performance hydrogenated acrylonitrile butadiene rubber and polyamide-12 EB crosslinked blend in comparison to the conventional crosslinked blend is reported by Reffai et al., [10].

The characteristic properties such as physico-mechanical, microstructure surface morphology, thermal stability, dielectric characteristics etc. of the radiation crosslinked elastomers predominantly depend on the optimum radiation dose [11]. Acrylic elastomer (AEM) is high heat and fluid resistance flexible rubber which makes it a significant elastomer to develop technically improved nanocomposites and blend with superior properties for various engineering applications.

However, the mechanical properties of AEM rubber are very poor in its virgin vulcanised form according to the application point of view; hence it needs to be compounded with reinforcing filler or blended with a compatible polymer with better mechanical properties [12, 13]. MPU is a thermosetting Polyurethane elastomer possessing excellent abrasion resistance, high elasticity, and high resistance to swelling with better tear and compression set properties as compared to TPU [14, 15]. The synergistic effect of AEM and MPU will make the blend a suitable material for the manufacturing of gasket which can be used in military, automobile and aerospace with different specification depending on the requirement. The seal made up of this material can also be used in the food and pharmaceutical sectors.

The prime objective of this study is to explore the effect of EB on the physico-mechanical and thermal properties of AEM/MPU blend. Before this investigation, the blend ratio is optimised using peroxide crosslinking system.

Experimental Technique

Material

AEM commercially known as Vamac DP having density 1.04 g cm⁻³ is procured from DuPont Performance Elastomers, Wilmington, Delaware, U.S.A. Polyurethane rubber (Urepan^® 50 EL 06 G) is obtained from RheinChemie Additives Germany. Di-Cup^® 40C and Vulcup TAC-70 (triallyl cyanurate) are obtained from Akzo Nobel NV are used as curatives.

Preparation of blends

The formulations of the blends are presented in Table 1 and the different ratios of AEM and MPU are mixed with other ingredients by using Haake PolyLab OS Rheomix followed by two-roll mixing mill (325 × 150) according to American Society for Testing and Materials (ASTM) D3I82 standards employing optimised nip gap, mixing time, and uniform cutting operation. The curing characteristics of the DCP and TAC (triallyl cyanurate) compounded mixtures are determined using a rubber process analyzer (RPA 2000) at 160°C as per the ASTM D 6601 and the calculated curing time is used for the fabrication of chemically crosslinked compression moulded sheets. The tensile characteristics of the compression moulded sheets are used as a major to obtain the optimised blend ratio and in the present study, it is found that the 60:40 blend exhibits highest tensile strength and hence this ratio is optimised one. The compression moulded sheets of 60:40 AEM/MPU blend (without DCP and TAC) have been prepared at 100°C with 10 MPa pressure using Hydraulics press having a thickness of about 2 mm.

Table 1

Formulation of DCP crosslinked systems.

Sl. No.	AEM (phr)	MPU (phr)	DCP(phr)	TAC(phr)
1	100	0.0	2.5	1
2	80	20	3.2	1.2
3	70	30	3.55	1.3
4	60	40	3.9	1.4
5	50	50	2.75	0.88
5	0.0	100	6	2

Electron beam radiation

The sheets having thicknesses of about 2 mm are subjected to EB irradiation of appropriate doses as mentioned in Table 2. The irradiation process is carried out using an industrial ILU-6 accelerator from Budker Institute of Nuclear Physics, Russia, with a set of conditions such as energy = 1.6 MeV, average current = 2.5 mA, pulse frequency = 25 Hz, under ascertained conditions at conveyor speed of 3 cm s⁻¹ which generates a dose of 12.5 kGy/pass as determined by nylon film dosimeter before irradiation.

Table 2

Radiation dose to AEM/MPU blend.

AEM:MPU (60:40)	EB Radiation Dose (kGy)
	50
	100
	200
	300
	400

Characterisations

Curing characteristics

The cure characteristics of the DCP and TAC compounded AEM/MPU blend is carried out at 160°C using RPA with the 3° arc of oscillation. The scorch time (T_S2), cure rate, delta torque and optimum curing time (T₉₀) are determined. (1) (2) where M_H= Maximum torque, M_L= Minimum torque, T_S2= scorch time (M_L+ 2 units)

Swelling and gel fraction

Circular testpieces with a radius of 20 mm were cut from a moulded sheet that was 1.5–2.0 mm thick. The accurately weighed samples are immersed in toluene at room temperature up to equilibrium swelling, and the swollen samples are weighed accurately. The volume swelling is calculated using the following Equation: (3) where, W_s is the weight of the swollen polymer and W_d is the initial weight of the polymer in dried condition

The gel fraction is calculated from the following relationship: (4) Here W₁ and W₂ are the initial and final weight after drying respectively.

Fourier-transform infrared spectroscopy (FTIR)

The Fourier-Transform Infrared (FTIR) spectra of the individual polymers and the polymer blend have been obtained using TSN with a resolution of 4 cm⁻¹ and range from 4000 cm⁻¹ to 650 cm⁻¹ at a 32 scan.

Scanning electron microscope (SEM)

The morphology of AEM: MPU (60:40) blend with 300 kGy were studied. The blend is gold coated with help of plasma sputtering then subjected to vacuum chamber of ZEISS Field Emission Scanning electron microscope at an accelerating voltage of 20 kV.

Crosslink density

The crosslink density was determined by equilibrium solvent (toluene) swelling measurement by the application of Flory–Rhener equation at room temperature. (5) where ν is the number of effective network chains, V_r is the volume fraction of polymer in the swollen mass, V_s is the molar volume of the solvent (106.2 g mol⁻¹ for toluene), and χ is the Flory–Huggins polymer–solvent interaction parameter. The value of χ is 0.4 for toluene. The volume fraction of polymer, V_r is calculated by the expression: (6) where T is the weight of the test specimen, F is the weight fraction of the insoluble components in the sample, D is the de-swollen weight of the test specimen, A₀ is the weight of absorbed solvent corrected for swelling increment, ρ_r is the density of the rubber (1.04 g cm⁻³ for AEM), and ρ_s is the density of solvent (0.87 g cm⁻³ for toluene)

Mechanical properties

The tensile strength of the vulcanised blend is measured using the universal testing machine (UTM) with a crosshead speed of 500 mm min^-1 under ambient conditions. The tensile properties were measured as per ASTM D 412. The blend with 60:40 (AEM: MPU) composition exhibited a highest tensile strength.

Thermogravimetric analysis (TGA)

The thermal stability of the EB crosslinked blend has been analysed using TGA (model Q50 V6.1 series, TA Instruments, Delaware) from room temperature to 700°C with a scan rate of 5°C min⁻¹ in dry nitrogen (N₂) gas atmosphere.

Results and discussion

Cure characteristics

The cure characteristics of the DCP compounded AEM and AEM/MPU blend has been shown in Figure 1 and Table 3. Irrespective of the blend ratio the curves attain a constant torque (M_H) and the improvement in viscoelastic nature with an increase in MPU loading confirms from the M_L values. The entanglement of AEM chain in the MPU phase which is the primary cause of the enhancement of the torque values. The significant improvement in the delta torque value by increasing the MPU loading is may be due to the restricted movement of the AEM polymeric chains in MPU phase [16, 17]. It is found that the optimum cure time (t₉₀) and the scorch time (t_s2) decrease with increase in MPU loading and the cure rate increases which is an important parameter since it determines the time in which the formation of three-dimensional crosslink networks in the compound is completed. The increase in stiffness of the systems with an increase in MPU loading is responsible for the increase in cure rate.

Figure 1

Cure Characteristics of DCP vulcanised AEM/MPU blend.

Table 3

Curing characteristics.

Sample	Minimum Torque (dNm)	Maximum Torque(dNm)	Delta Torque (dNm)	Scorch time (min.)	Cure Rate (1/min)	Optimum curing rate (min.)
AEM	1.33	22.89	20.83	3.24	7.70	16.22
80:20	4.22	27.44	25.12	1.56	8.28	12.45
70:30	6.30	41.49	37.97	1.30	8.53	13.01
60:40	7.21	49.39	45.17	1.20	8.72	12.66
50:50	9.11	59.10	50.01	1.10	9.93	11.17

Swelling and gel fraction

The interaction between the two polymeric phases in a blend can be determined from the solvent swelling studies. The swelling analysis of DCP vulcanised AEM and AEM/MPU blends are shown in Figure 2 and it is found that AEM shows highest swelling characteristics and it follows reducing trend by increasing the MPU loading. As observed from the curing characteristics, the crosslink networks increase with an increase in MPU loading through the formation of three-dimensional crosslinking networks which reduces the free space in the system that restricts the entrapment of the solvent molecules. These above findings can be interpreted as the inertness of the systems increase towards the solvent, which results in the reduction of swelling percentage and increase in the insoluble component that attributes towards higher gel fraction.

Figure 2

Swelling and Gel Fraction of DCP vulcanised AEM/MPU blend.

Mechanical properties

The mechanical properties, such as tensile strength (TS) and elongation at break (EB %) of DCP vulcanised AEM gum, MPU gum and AEM/MPU blends as a function of MPU loading have been shown in Figure 3. It has been observed that AEM and MPU exhibit 0.06 and 1.18 MPa tensile strength values respectively. However, a significant improvement in tensile strength result is observed in case of DCP crosslinked AEM/MPU blend and among the blend formulations, the 60:40 AEM/MPU blend exhibits the highest TS value, this confirms the existence of well interfacial adhesion and better compatibility between the components up to this loading level. However, 50:50 blend ratios follows a decreasing trend which may be due to the lack of interfacial adhesion and prominent phase separation between the two phases of the blend. [2, 18]. It has been observed that the EB% of the AEM/MPU blends also decreases with increasing the MPU content and the factors which are responsible for this decremental behaviour in crosslinking and stiffness of the systems.

Figure 3

Mechanical Properties of DCP vulcanised AEM/MPU blend.

Scanning electron microscope

The surface morphology and the interactions between the two phases of the blend in terms of interfacial adhesion and crosslinking have been analysed by scanning electron microscopy technique. Figure 4 shows the SEM images of tensile fractured surface morphology of DCP crosslinked and EB crosslinked of 60:40 AEM/MPU blend. The effortless tensile failure generates a smooth surface morphology which indicates the lack of interfacial adhesion in case of blending systems. Based on this concept, the Figure 4(a,d) which are the surface morphology obtained of DCP crosslinked and crosslinked done with 400 kGy radiation dose exhibit smooth tensile fracture morphology that indicates the poor interfacial adhesion [19, 20] and radiation-induced degradation, respectively. However, the surface roughness observed in Figure 4(b,c) for 60:40 blend irradiated in the radiation dose of 200 and 300 kGy significantly explains the establishment of enormous crosslinked network structures in the interfacial region. This finding can be well correlated with the tensile result discussed earlier in the above section.

Figure 4

SEM microphotographs of (a) 60:40 DCP vulcanised (b) 60:40 @ 200kGy (c) 60:40 @ 300 kGy (d) 60:40 @ 400 kGy blend.

Fourier-transform infrared spectroscopy

Figure 5 represents FTIR spectra of AEM, MPU, and AEM/MPU blend. The carbonyl stretching frequencies of the ester group for both the polymers is observed at 1732 cm⁻¹ and in case of the blend of these two components, the peak for carbonyl stretching is found in the frequency range of 1727 cm⁻¹. In the case of blends of the two functional polymers, there must be chemical interaction established between the two components. In the present study, the shifting of the carbonyl group peak position from 1732 to 1727 cm⁻¹ illustrates the establishment of the hydrogen bonding between the hydrogen atom of –NH₂ group of MPU with the = CO group(3)of AEM.

Figure 5

FTIR spectra of AEM, MPU, and AEM/MPU blend (60:40).

Crosslink density, swelling and gel fraction

From Figure 6(a), it has been observed that the crosslink density of EB crosslinked AEM/MPU blend increases with an increase in radiation dose. At the higher dose of radiation, more generation of reactive free radicals have taken place, and the combination of these free radicals caused the formation of three-dimensional crosslinked networks in the interfacial area of the blend which results in an increase in crosslink density [21, 22].It is clearly observed from Figure 6(b) that the swelling value decrease with an increase in radiation dose. These phenomena occur because radiation accelerates the formation of three-dimensional crosslinked networks in the elastomeric system which opposes the entrapment of solvent molecules into the systems results in restriction swelling [16]. Figure 6(b) also represents the variation in the gel fraction of the EB crosslinked AEM/MPU blend. Moreover, it has been observed that the gel fraction increases with increase in radiation dose and this may be correlated with increased in the extent of crosslinking with a radiation dose which increases the network structures in the system [8, 13].

Figure 6

(A) Crosslink Density (B) Swelling and Gel Fraction of AEM/MPU (60:40) Blend at different radiation dose.

Mechanical properties

In case of irradiated systems up to 200 kGy radiation dose a significant improvement in the tensile result is found, which is represented in Figure 7 and that is only for the establishment of crosslinked network structures in the interfacial region as observed in crosslink density analysis and tensile fractured surface morphology. However, above this radiation dose, the tensile result shows a decremental trend, which is due to the radiation-induced degradation phenomenon that leads to the formation of highly crosslinked mini clusters with very small chain length. The mini clusters are unable to dissipate the applied tensile force through inter-component, intra-component process in the irradiated matrix [4, 23]. From the above findings it can be interpreted that, on irradiation of 60:40 blend, both radiation-induced crosslinking and degradation processes take place simultaneously but with different rates. However, in the current studies, the degradation process is apparently significant above 200 kGy radiation dose. Apart from that the oxidative degradation of the elastomer at relatively higher radiation dose could be another factor which leads to the decreasing trend of tensile value. Highly crosslinked networks at the interface are mainly responsible for the decrease in segmental mobility and an increase in the stiffness of the systems which reflects in the result in terms of decrease in EB% [24].

Figure 7

Tensile Properties of EB crosslinked AEM/MPU blend.

Thermal stability

The TGA thermograms of AEM, MPU, 60:40 DCP vulcanised and 60:40 blends irradiated with a different dose of EB are interpreted in Figure 8. From the degradation curve it has been observed that AEM and MPU show their initial degradation around 370^o and 205°C respectively. The DCP vulcanised 60:40 blend shows better thermal stability than MPU matrix which exhibits the initial degradation around 336^oC and this is because of the development of network structures in both the components as well as in the interface of the blend through the free radical reaction mechanism. However, the thermal stability of 60:40 irradiated blends significantly increases with an increase in radiation dose. Crosslink density is a prime factor which affects significantly the thermal stability of the crosslinked rubber. High degree of crosslinking network results in better thermal stability because of the higher value of activation energy required for the thermal decomposition of EB curing AEM/MPU blends [9, 23]. Thus, radiation curing improves the thermal stability of the blends.

Figure 8

Thermogravimetric analysis.

Conclusion

The DCP vulcanised AEM/MPU blend ratio has been successfully optimised which is found to be 60:40 on the basis of tensile results which gives a clear sight of compatibility and adequate interfacial adhesion. The optimised blend ratio has been exposed to different EB dose and the optimised EB dose is obtained on the basis of their improvement in mechanical characteristics. The interfacial adhesion between the two components of the DCP vulcanised as well as irradiated has been observed using SEM photo micrographs. The establishment of hydrogen bonding between the two components is confirmed from the FTIR result. The formation of three-dimensional networks is confirmed from the swelling, gel fraction and crosslink density results. The irradiated blend shows an incremental trend in TS value up to 200 kGy, however the EB% is getting decreased with irradiation dose due to the increase in stiffness. From the TGA it has been confirmed that the thermal stability of EB crosslinked blend is superior to that of the DCP vulcanised blend. From the above studies it can be suggested that the blend can be used for the fabrication of conductive blend nanocomposites by incorporating conductive nanofillers such as graphene, carbon nanotubes and other metal oxides particularly of electromagnetic radiation shielding applications. Also the irradiated blend can use directly for cable seathing application.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the authors.

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