Exploring the influence of electron beam irradiation on the morphology, physico-mechanical, thermal behaviour and performance properties of EVA and TPU blends

Abstract

The effects of electron beam radiation (EBR) on the blends of ethylene vinyl acetate/thermoplastic polyurethane (EVA/TPU) at two different blend ratios prepared via melt blending technique were investigated. All the samples were irradiated by using a 2.5 MeV electron beam accelerating energy over a dose range from 25 to 200 kGy. The blends exhibit drastic improvement in the mechanical properties with increasing radiation dose upto an optimum dosage, beyond which the properties began to deteriorate. Modification of the blends via EBR enhances the elastic recovery of the blends resulting significant improvement in tension set behaviour. DSC study shows that electron beam irradiation causes a marginal change in melting temperature (T_m), glass transition temperature (T_g) and crystallization peak temperature (T_c) of EVA/TPU blends. Dynamic mechanical analysis (DMA) was conducted to investigate the change of loss tangent and storage modulus with varying radiation dose. Thermogravimetric analysis (TGA) suggests that irradiation induced crosslinks also help to improve the thermal stability of the blends to some extent. Scanning electron microscopy (SEM) study was performed to explore the changes in morphology before and after irradiation. All the irradiated blends have higher electrical resistivity than the blends without irradiation and the volume resistivity increases upto 150 kGy. The samples were found to exhibit remarkable improvement in oil resistance property after irradiation which is more prominent in EVA/TPU 70/30 blends.

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Introduction

Blending of polymers that can generally affect the physico-mechanical and chemical behaviour of two or more immiscible blending components has become an intriguing and thriving area of research for scientific and commercial purposes. Electron beam (EB) irradiation has been found to be a useful and well-known tool for the improvement of the mechanical properties of polymers.^1,2 Electron beam treatment has several advantages over conventional chemical or heat treatment processes. EB process is faster, economically viable than chemical crosslinking and less hazardous as it excludes the generation of unwanted by-products (like toxic and harmful peroxide residue in case of peroxide curing). Moreover, EB radiation technique helps to maintain the purity of the processed product as it does not require any catalyst or solvent and this process occurs at ambient temperature which further excludes the possibility of thermal degradation for thermally sensitive components. It not only reduces scrap and waste but the doses of irradiation can also be controlled easily.^3,4 The irradiation induces enormous reactive species like radicals and ions leading to cross-linking or degradation, the former being more dominating and pronounced depending mainly on the chemical structure of the polymers and the irradiation dose applied. On exposure to electron beam radiation, chemical bonds are formed between polymer molecules to produce a three dimensional insoluble network which in turn imparts superior physical properties, chemical and solvent resistance, thermal and dimensional stability of the polymer blend system. All these changes lead to huge scope for industrial application. However at very high radiation doses excessive crosslinking often makes the polymer brittle and the material fails to serve for industrial purpose.^5,6 Electron beam curing of polymer has been carried out in a wide range of fields, for example in electrical wire and cable application, coating and packaging industries etc.

EVA is a random copolymer containing polar vinyl acetate (VA) and nonpolar ethylene monomers in main chain and by varying the VA content they can be tailored for application as rubber, thermoplastic elastomer and plastic. EVA exhibits excellent aging and weather resistance, good electrical insulation and remarkable mechanical properties which make this polymer suitable for various industrial applications like cable insulation and sheathing material, flexible packaging, photovoltaic encapsulation, hose, tube and footwear. Work on electron beam radiation of different polymer and their blends over the last few decades have revealed that radiation curing have got the potential to play an important role in modification of polymer blend systems.^7–10 EVA has the ability to form crosslinked network even at low radiation doses with consecutive improvement in mechanical and thermal properties.^11,12 The influence of low, medium and high energy radiation on mechanical, thermal and electrical properties of EVA/low density polyethylene (LDPE) blend system have been extensively studied by several group of researchers.^13–17 Chattopadhyay et al. have investigated about the heat shrinkability of irradiated EB cured EVA/polethylene blends¹⁸ and Dalai's group have successfully produced crosslinked foam from irradiated EVA/LDPE blends containing azodicarbonamide.¹⁹ Hui and Xu showed that irradiated EVA/high density polyethylene blends exhibit better flexibility, heat aging and memory effect and found the material suitable for cable insulation and heat shrunk connectors.²⁰ It has been observed that on exposure to electron beam radiation EVA exhibits superior crosslinking efficiency as compared to LDPE.⁴ Dalai and co-workers also reported that higher EVA content in the blend is favourable for improving the radiation crosslinking in EVA/polypropylene blends irradiated by gamma ray in air.²¹ Zurina et al. discussed about the morphology, physical and dynamic mechanical behaviour of irradiated EVA/epoxidised natural rubber (ENR 50) blend and improved their thermal stability.²² Wang et al. showed that incorporation of EVA with higher vinyl acetate content significantly enhances the mechanical properties and gel content of EVA/polyvinyl chloride blends upon irradiation.²³

Thermoplastic polyurethane (TPU) belongs to the family of materials known as thermoplastic elastomers (TPE). It is a linear segmented block copolymer having alternating hard (adduct of di-isocyanate and small glycols) segments with a high melting point and soft (e.g. polyester, polyether etc.) segments with a high extensibility and low glass transition temperature and these segments are connected to each other by the urethane groups (–NH–COO–). By altering the ratio of the hard and soft segments, it is possible to vary the properties over a broad range. TPUs can offer a number of superior physical properties like high tensile modulus, resilience, abrasion resistance, wear and tear resistance, low-temperature flexibility, good compression set, as well as excellent chemical and solvent resistance.^24–26 TPU being a relatively expensive material, the interest of the authors was to produce a EVA/TPU blend with a new set of attracting properties using less amount of TPU. In our previous paper a novel blend based on EVA and TPU at different ratios were prepared and studied in details.²⁷ EVA/TPU 80/20 blend was found to show the optimum physico-mechanical properties and the mechanical properties gradually diminished as the TPU content increases upto 50%. However it was also observed that incorporation of even small amount of TPU significantly improves the oil resistance property of the blend which is an important criteria for cable sheath material. It is also to be noted that in spite of being a low cost material with numerous good qualities, EVA has the ability to be compounded with large amount of additives and can easily form crosslinked network compared to many other polymers even at lower radiation doses. However, to the best of our knowledge no systematic studies have been made so far on the electron beam irradiated EVA (28% VA content) and TPU blend system. Thus considering the earlier results EVA/TPU 80/20 and 70/30 blends have been chosen for further studies on electron beam curing. The primary objective of the present research work is to study the influences of electron beam irradiation on morphology, mechanical, thermal, and dynamic mechanical properties of the blend system in detail and to develop an economically viable useful blend system. The volume resistivity and oil resistance property of the electron beam treated blends have also been explored. Hence, our aim is to modify the properties of the EVA/TPU blend system with the help of low energy electron beam irradiation in a cost effective and less hazardous way for technological applications.

Experimental

Materials

Thermoplastic Polyurethane (TPU) Desmopan 385 S, composed of 4,4′-diphenylmethane diisocyanate hard segment and polyester based soft segment was supplied by Bayer Chemicals, India. It has a density of 1.2 g cm⁻³ with a Shore-D hardness of 34 and its melting temperature is around 170 °C. Ethylene vinyl acetate copolymer (EVA) containing 28% vinyl acetate, grade Elvax 265, with a density of 0.95 g cm⁻³ and a melt flow index (MFI) of 1.7 g/10 min, was procured from Dupont, India.

Preparation of blends

EVA/TPU blends were mixed in Haake Rheomix OS (Germany) 600 internal mixer, having a mixing chamber volume of 85 cm³ at 180 °C for 7 min. At first TPU was added and it was allowed to melt for 2 min at a rotor speed of 70 rpm. After that EVA was charged into the mixing chamber and the mixing was continued for further 5 min. The mixes so obtained were sheeted under hot conditions in an open mill set at 2 mm nip gap. The sheets were then compression molded between Teflon sheets for 4 min at 190 °C at a pressure of 5 MPa in an electrically heated hydraulic press (Moore Hydraulic Press, England) to produce about 2 mm thick sheets. After that the moulded samples were cooled under pressure to maintain the overall dimensional stability of the moulded articles. EVA/TPU 80/20 blend and EVA/TPU 70/30 blends have been denoted as ET80 and ET70 respectively.

Irradiation of samples

The molded sheets were irradiated in air at room temperature using an electron beam accelerator (Model: Dynamitron, 369 supplied by RDI, USA) at Nicco Corporation limited, Kolkata, India. Irradiation doses of 25, 50, 75, 100, 150, and 200 kGy have been used for this study. The acceleration energy and beam current were 2.5 MeV and 17 mA respectively. Here, the electron beam irradiated samples designated as ET70R100 indicate EVA/TPU 70/30 blend radiated at 100 kGy radiation dose.

Characterization of blends

Mechanical tests. Tensile test was carried out using a universal testing machine Hounsfield H10KS at room temperature at a crosshead speed of 200 mm min⁻¹. Tensile specimens were punched from the moulded sheets using ASTM Die-C as per ASTM D 412. Three measurements were taken for each samples and an average of results was reported as the resultant value.

Tension set test. For tension set measurement, the dumbbell specimens were extended upto 100% in the tensile direction at a rate of 200 mm min⁻¹ and kept at that position for 10 min at room temperature. It was then relaxed back to unstressed condition and the percentage change in dimension in tensile direction was measured after 15 min and reported as tension set.

Hardness test. Hardness of the samples was measured in Shore-D scale as per ASTM D2240 standard at room temperature using Shore D hardness of samples were obtained by a Shore-D hardness-testing machine (Bowers Mertrology, UK) as per ASTM D2240 standard at room temperature. The hardness value is determined by the penetration of the Durometer indenter foot into the sample.

Dynamic mechanical thermal analysis (DMA)

DMA analyses of the samples were carried out using a dynamic mechanical analyzer, Eplexor 150N DMTA (Gabo Qualimeter, Ahlden, Germany). Tests were carried out at a frequency of 10 Hz under a static strain of 0.50% and a dynamic strain of 0.001% over a temperature range of −100 °C to 80 °C. The samples were first cooled to −100 °C and then subsequently heated at a rate of 2 °C min⁻¹. The temperature corresponding to the peak in tan δ versus temperature plot was taken as the T_g.

Differential scanning calorimetry (DSC)

Differential Scanning Calorimetry studies of the pure polymers and the blends were carried out using a DSC Q2000 (TA Instruments, USA) in an inert atmosphere (N₂ atmosphere) at a heating and cooling rate of 10 °C min⁻¹. The experiment was conducted from −80 °C to 230 °C for all the samples. Samples weighing about 10 mg were taken and standard aluminium pans were used to analyze the samples. Glass transition temperatures (T_g) and melting behaviour were observed from the second heating run of DSC plot. The data of second heating cycle was used to eliminate thermal history.

Morphological study

Surface morphology of the blends was examined by using JEOL JSM 5800 Digital Scanning Electron Microscope (SEM). The accelerating potential 20 kV was used for the analysis of sample. All the blends were cryofractured in liquid nitrogen to avoid any possibility of phase deformation during cracking process. The cryofractured surface of the blends was etched in tetrahydrofuran (THF) solvent for 1 day in order to remove TPU phase of the blends. The etched surface after adequate drying for 24 hours at room temperature was gold sputtered and then observed under SEM.

FTIR spectroscopy

FT-IR spectra on the thin films of the polymer blends before and after electron beam irradiation were recorded using a Perkin-Elmer Frontier spectrometer (UK) in ATR mode at room temperature over the range of 4000–400 cm⁻¹.

Thermogravimetric analysis (TGA)

Thermogravimetric analyses (TGA) and derivative thermogravimetry (DTG) of the neat components as well as EVA/TPU blends were measured by using a thermogravimetric analyzer (Mettler-Tolledo AG, Switzerland). The sample weight was 8–10 mg and the heating rate was 10 °C min⁻¹. Tests were performed from ambient temperature to 700 °C under N₂ atmosphere.

Gel content

The gel content of the crosslinked samples was determined by the extraction of the sol components in boiling xylene for 24 hours using a Soxhlet apparatus. Three samples were used in each case and the average was taken.where, W₁ and W₀ are the weight of the dried sample after extraction and the weight of the sample before extraction, respectively. The extracted samples were dried in oven at 50 °C until they attained a constant weight.

Crosslink density measurement

Crosslink density of the irradiated samples was determined on the basis of equilibrium solvent swelling measurements in xylene at room temperature for 1 week. At the end of immersion period the sample was removed, gently wiped with tissue and transferred to the weighing balance to obtain the swollen weight of the sample. From the degree of swelling cross-link density (CLD) of EVA in presence of TPU was calculated by using Flory–Rehner equation.^28–30where, ν = number of moles of effectively elastic chains per unit volume of EVA [mol ml⁻¹] (crosslink density), V_s = molar volume of solvent (xylene) [cm³ mol⁻¹], χ = Flory–Huggins interaction parameter for polymer-swelling agent (about 0.29) at 30 °C as obtained from literature.³¹ V_r is the volume fraction of rubber in the swollen network and V_r can be expressed as eqn (4):where, A_r = ratio of the volume of absorbed solvent (xylene) to that of EVA after swelling.

Volume resistivity test

Volume resistivity of the samples (dimension 10 × 10 cm²) has been measured in a Hewlett Packard 4339B (manufactured by Agilent Technology, Japan) high resistance meter at room temperature (30 °C) with an applied voltage of 500 V. Volume resistivity has been calculated using the following formula and measured as per ASTM D-257-66. The specimen was placed in the resistivity cell, clamped between electrodes and guard ring, for the specified time and applied voltage.where, A = the area of upper electrode (19.6 cm²), R = the resistance (in ohm) between upper and lower electrode t = the thickness (in cm) of the test specimen.

Oil swelling study

The test specimens were immersed in ASTM 3 oil at room temperature for 7 days. After the required period of time the specimens were removed from the oil, quickly dipped in acetone, and blotted lightly with a clean blotting paper to eliminate the excess oil on the specimen surfaces and the final weight was taken. For every single composition three specimens were tested and their average values have been reported. Swelling percent was measured as follows:where, M₁ = initial mass of specimen in air and M₂ = mass of specimen in air after immersion.

The percentage error in the oil swelling was found to be more or less ±1.5%.

Results and discussion

Physico-mechanical properties

Physico-mechanical properties, such as, tensile strength, elongation at break, modulus at different percentage of strain, hardness and tensile set of the EVA/TPU blends at varying radiation doses are shown in Table 1. From Fig. 1 it has been clearly observed that dramatic improvements took place in the tensile properties upon irradiation. On exposure to EBR at a dose of only 25 kGy, the tensile strength (T.S.) increases by 45% and 24% for ET 70 and ET 80 blends, respectively. With increasing radiation dose from 25 kGy to 100 kGy, T.S. of both the blends sharply increases but beyond 100 kGy upto 150 kGy, there is marginal improvement of T.S. upto 150 kGy and at 200 kGy, T.S. value slightly decreases. Such remarkable improvement in tensile strength can be attributed to the formation of radiation induced crosslinked network and some interfacial interaction which reduces the probability of formation and propagation of crack at interfaces during tensile stretching.^13,14 Ionizing radiation either leads to the formation of chemical crosslinks between the molecular chains or causes degradation or chain scission which destroys the molecular structure. Ionizing radiation causes the formation of chemical crosslinks between the molecular chains and degradation or chain scission which destroys the molecular structure. Although both the process occur simultaneously upon irradiation, at very high radiation doses chain scission become more pronounced¹³ and it is found that dose above 150 kGy can initiate some degradation for EVA/TPU blend system leading to deterioration of mechanical properties. It can be said that at higher radiation doses the main chain backbone of the polymer may break into a number of small fragments, which cause the formation of small clusters of crosslinked network. Such fragments consist of a larger number of polymer molecules crosslinked tightly; however they may act as separate physical entities. The intercluster force may not be sufficient to hold them under externally applied separating forces during tensile test.³²

Table 1 Physico-mechanical properties of different blends before and after irradiation at various doses

Sample designation	Radiation dose (kGy)	Tensile strength (MPa)	EB%	Mod at 100% (MPa)	Mod at 200% (MPa)	Mod at 300% (MPa)	Hardness (Shore D)	Tension set (%)
ET 80	0	17.6 ± 0.4	1067 ± 45	3.3 ± 0.2	3.9 ± 0.3	4.3 ± 0.2	25.2 ± 0.3	17 ± 0.5
	25	21.8 ± 0.6	849 ± 40	3.6 ± 0.2	4.4 ± 0.2	5.6 ± 0.3	27.1 ± 0.2	16 ± 0.6
	50	24.2 ± 0.4	822 ± 27	3.7 ± 0.1	4.5 ± 0.2	5.8 ± 0.3	28.2 ± 0.2	14 ± 0.4
	75	25.7 ± 0.3	797 ± 16	3.8 ± 0.2	4.6 ± 0.1	6.1 ± 0.1	29.1 ± 0.4	13 ± 1.0
	100	26.7 ± 0.5	754 ± 31	4.0 ± 0.2	4.8 ± 0.3	6.3 ± 0.2	30.0 ± 0.2	11 ± 0.8
	150	27.0 ± 0.3	742 ± 25	4.1 ± 0.1	5.4 ± 0.4	7.0 ± 0.2	31.3 ± 0.2	11 ± 0.4
	200	25.6 ± 0.4	661 ± 22	4.1 ± 0.3	5.6 ± 0.2	7.2 ± 0.1	31.6 ± 0.3	10 ± 0.5
ET 70	0	12.2 ± 0.6	832 ± 25	3.5 ± 0.3	4.0 ± 0.2	4.5 ± 0.2	25.5 ± 0.2	18 ± 0.6
	25	17.7 ± 0.7	868 ± 32	3.8 ± 0.1	4.7 ± 0.3	5.9 ± 0.4	27.6 ± 0.2	16 ± 0.8
	50	22.1 ± 0.4	826 ± 48	3.9 ± 0.2	4.9 ± 0.2	6.3 ± 0.3	28.4 ± 0.2	15 ± 1.0
	75	24.8 ± 0.5	754 ± 42	3.9 ± 0.2	5.1 ± 0.1	6.7 ± 0.2	29.5 ± 0.3	14 ± 0.5
	100	26.4 ± 0.6	738 ± 17	4.0 ± 0.2	5.2 ± 0.3	7.1 ± 0.2	30.3 ± 0.4	12 ± 0.4
	150	26.8 ± 0.3	727 ± 12	4.1 ± 0.1	5.4 ± 0.2	7.4 ± 0.2	31.4 ± 0.3	12 ± 0.3
	200	24.9 ± 0.3	610 ± 23	4.2 ± 0.1	5.5 ± 0.2	7.5 ± 0.1	32.3 ± 0.2	11 ± 0.7

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Fig. 1 Variation of tensile strength with radiation dose.

From Table 1 it is observed that for ET70 blend, elongation at break (EB%) slightly increases from 832% (in case of non-irradiated blend) to 868% at 25 kGy and thereafter, it gradually decreases with increasing radiation dose and becomes 610% at 200 kGy, a reduction by 27% from the non-irradiated blend. Although EB% of non-irradiated ET80 blend has a quite high value of 1067%, it continuously decreases upon irradiation. EB% value has been found to drop by 29.3% in case of ET80 blend at 100 kGy. Upon further irradiation at 200 kGy, it decreases to 661% which is almost 35% less than that of non-irradiated blend. Such decrease in EB% can be assigned to the formation of three-dimensional network structure upon irradiation which prevents the structural reorganization during elongation.³³ As a consequence of increased crosslink density (CLD), the internal chain mobility is somewhat restricted against the external applied force and hence, the blend shows lower value of EB.

It has been observed that for all the blends, modulus at 100% strain gradually increase with radiation dose. As the radiation dose increases from 25 to 200 kGy the modulus at 100% elongation changes from 3.6 MPa to 4.1 MPa for ET80 and in case of ET70 it varies from 3.8 MPa to 4.2 MPa. It should also be noted that modulus at 200% and at 300% strain exhibit similar improvement for both the blends. Fig. 2 depicts the change in modulus at 300% as a function of radiation dose. 300% modulus has been found to show an increase of 66% and 67% at 200 kGy for ET70 and ET80 blends respectively with respect to their unirradiated blends. The modulus depends directly on the degree of crosslinking, whereas T.S. and elongation behaviour are dependent on two competing factors of crosslinking and strain hardening.³⁴ Thus the increase in CLD is also reflected from such improvement in modulus with radiation dose.

Fig. 2 Variation of modulus at 300% elongation as a function of radiation dose.

The results of tension set test are shown in Table 1. It is a well known fact that lower set % value indicates better elastic recovery and Table 1 clearly shows that the tension set value of both the blends reduces on exposure to electron beam radiation. Tension set value for ET70 and ET80 blend decrease from 18% and 17% to 12% and 11% respectively at 100 kGy. However, no significant improvement in the set values was noticed with further increase in radiation dose. Since radiation induced crosslinking enhances the elastic recovery of polymer, tension set property of all EVA/TPU blends improve with increasing radiation dose.

Dependence of hardness upon radiation dose has been reported in Table 1. It is observed that hardness of the samples gradually increase with radiation dose upto 200 kGy. The hardness of non-irradiated ET 80 blend was only 25.2 and it becomes 31.6 at 200 kGy whereas for ET70 hardness increase from 25.5 to 32.3 in shore D scale. It is evident that such enhancement in hardness occurs due to the formation of crosslinked network which has also been confirmed from the gel content test. Hardness is usually referred to the local deformation and the above result clearly indicates that the irradiated samples have better resistance to local deformation resulting increase in hardness values.

Altogether it has been found that 100 kGy radiation dose imparts the best combination of physico-mechanical properties for EVA/TPU blend system.

Dynamic mechanical analysis

Dynamic mechanical analysis (DMA) has been widely used for investigating the visco-elastic behaviour of polymer blends in order to determine their relevant stiffness and damping characteristics for various applications. The dynamic mechanical properties for the non-radiated and irradiated EVA/TPU blends were analysed in the temperature range from −80 °C to +60 °C. The variation of storage modulus (E′) and tan δ for ET80 and ET70 blends with radiation dose has been reported in Table 2. It has been found that for the irradiated blends there is an increase in storage modulus value with increase in radiation dose upto 150 kGy. For ET80, the E′ value steadily increases from 1532 MPa to 2058 MPa at −60 °C, as the radiation dose increases from 0 to 150 kGy and under similar conditions in ET70 the E′ value changes from 1645 MPa to 2160 MPa. Although for both the blends storage modulus marginally decreases as the dose reaches 200 kGy. Similar trend in storage modulus has also been observed at ambient temperature. The initial rise in storage modulus is associated with the crosslinking induced by irradiation which consequently improves the interfacial adhesion and impart stiffness to the blends. However, at much higher radiation dose (here 200 kGy) the reduction in storage modulus is probably due to some oxidative degradation which cause the rupture of crosslinked network to some extent leading to reduction in stiffness of the matrix.³⁵ Fig. 3a and b represent the influence of radiation dose on tan δ of ET80 and ET70 blends respectively. The glass transition temperature of the blends can be obtained from the maximum peak temperature of tan δ plot. On exposure to EBR (the position of tan δ max peak corresponds to T_g) T_g of the blends gradually shifts to higher temperature but the change is only marginal. The T_g value increases by about 3–4 °C on an average as the radiation dose reaches 150 kGy. Such behaviour can be attributed to the fact that the crosslinks formed between the chains restrict the molecular motion and hence more energy is required for glass to rubber transition.³⁶ Here also at 200 kGy the T_g value has been found to shift slightly to lower temperature which indicates that at 200 kGy radiation dose, the occurrence of some chain scission process cut short some of the polymer chains into smaller fragments leading to relatively easier mobility of chain segments compared to 150 kGy. It is also to be noted that there is slight reduction in the broadening of tan δ peak for the irradiated ET80 and ET70 blends that suggests that somewhat better compatibility was achieved by electron beam radiation method.²² This fact can also be supported further from the mechanical property improvement and morphology. The progressive drop in tan δ max peak height for both the blend system indicates that the crosslinked network causes hindrance to the molecular motion and as a result of this there is depletion in viscous dissipation or mechanical loss energy.^6,37

Table 2 Dynamic mechanical properties of different blends irradiated at various doses

Sample designation	Radiation dose (kGy)	tan δ peak (°C)	tan δ (max)	E modulus at −60 °C (MPa)	E modulus at 25 °C (MPa)
ET 80	0	−15.88	0.329	1532	19.54
	25	−15.37	0.325	1645	23.84
	50	−15.07	0.321	1805	24.27
	75	−14.76	0.316	1820	24.73
	100	−14.27	0.310	1989	25.12
	150	−12.81	0.298	2058	29.10
	200	−13.79	0.303	2010	26.92
ET 70	0	−16.68	0.334	1645	24.27
	25	−15.73	0.328	1885	26.32
	50	−15.17	0.322	1922	26.44
	75	−14.62	0.318	1973	27.12
	100	−14.08	0.313	2011	27.21
	150	−13.02	0.300	2160	29.83
	200	−13.88	0.304	2141	29.70

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Fig. 3 DMA tan δ as a function of temperature at various radiation doses for (a) ET80 and (b) ET70.

Differential scanning calorimetric (DSC) studies

The DSC heating and cooling scans were conducted to find out the melting temperature (T_m), glass transition temperature (T_g) and crystallization peak temperature (T_c) for different blends as a function of radiation dose and the results are shown in Table 3. Fig. 4a and b demonstrate the second heating curves at various doses for ET80 and ET70 respectively. It has been found that the T_g of the blends changes only marginally with irradiation. The slight increment in T_g value after EBR can be associated with the formation of three dimensional network in EVA matrix. Higher crosslink density indicates shorter chain length between the crosslinking points which restrict the segmental mobility leading to an increase in T_g.³⁸ Melting temperature (T_m) of the EVA/TPU blends against radiation doses has been enlisted in Table 3. T_m was found to decrease steadily with increasing radiation dosage. It is clearly observed that unirradiated ET80 and ET70 blends have the higher melting temperatures which decrease by almost 7 °C as the electron beam dose reaches 200 kGy. This is because of reduction in concentration of segments of a length suitable for crystallization³⁹ and the melting temperature of EVA seems to be more affected by EB radiation because of its higher crosslink density. This pattern is similar for all the samples irradiated at various doses. The influence of irradiation on crystallization peak temperature (T_c) as reported in Table 3 reveals that the T_c corresponding to EVA phase continuously diminishes with increasing radiation dose upto 200 kGy. This lowering of T_c results from the lower degree of chain alignment due to predominant crosslinking, which hinders the growth of the crystals and consequently requires more undercooling to crystallize.⁴⁰

Table 3 Results of DSC analysis of various samples at different radiation doses

Sample code	Radiation dose (kGy)	Glass transition temperature, Tg (°C)	Melting temperature, Tm (°C)	Crystallization temperature, Tc (°C)
ET 80	0	−29.12	77.03	55.01
	25	−28.76	74.76	53.48
	50	−28.64	73.34	52.33
	75	−27.64	73.32	51.82
	100	−27.44	72.15	50.64
	200	−28.34	70.59	50.25
ET 70	0	−31.02	77.64	55.03
	25	−29.85	76.05	54.07
	50	−29.21	73.40	53.45
	75	−28.92	72.24	52.84
	100	−28.24	71.95	51.33
	200	−29.67	70.84	50.68

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Fig. 4 DSC second heating curves blends at various radiation doses for (a) ET80 and (b) ET70.

Morphology

Fig. 5 and 6 represent the SEM photomicrographs of ET70 and ET80 blends respectively. The samples were cryofractured in liquid nitrogen and then etched with tetrahydrofuran to preferentially remove the TPU phase, the minor component of the blends and the black domains indicate the positions of the extracted TPU phase. Fig. 5a–d shows the morphology of EVA/TPU 70/30 blends in non-irradiated and irradiated (at different irradiation doses) conditions respectively. SEM photomicrographs reveal the development of two phase morphology for all the samples and it can be observed that TPU phase can be extracted in presence of THF, even after the electron beam treatment which implies that the minor TPU phase in the blends cannot form effective crosslinked structure under such experimental condition; otherwise it would become very difficult to remove the TPU phase from the blends after irradiation.³² Similar kind of variation in morphology has also been found for EVA/TPU 80/20 blends as depicted in Fig. 6a–d. However, at the same time the TPU domain size becomes relatively smaller with electron beam dosage as compared to the un-irradiated blends. It can be said that with increasing radiation dose some interfacial crosslinks also formed between the EVA matrix and TPU domain that restrict the easy removal of TPU phase to some extent due to better interfacial adhesion and improved compatibility. On exposure to electron beam radiation, its powerful energy may also help to deform and shear cut TPU in EVA matrix which may also contribute in reducing the size of TPU domain.²²

Fig. 5 SEM photomicrographs of the THF extracted fractured surfaces of (a) ET70R0 (b) ET70R50, (c) ET70R100 and (d) ET70R150.

Fig. 6 SEM photomicrographs of the THF extracted fractured surfaces of (a) ET80R0 (b) ET80R50, (c) ET80R100 and (d) ET80R150.

Fig. 7A and B demonstrate the plausible schematic diagram for EVA/TPU blends before and after irradiation respectively. Fig. 7A shows the dispersion TPU domains in EVA matrix whereas Fig. 7B depicts the intermolecular crosslink formation in EVA matrix and TPU domain as well as some interfacial crosslinks between EVA and TPU. It is noteworthy to mention that although there is a more compact dense network formation within the EVA matrix, the extent of crosslink formation is relatively much lower in TPU domain with respect to EVA. It indicates that 25 to 200 kGy radiation dose with 2.5 MeV electron beam acceleration energy and beam current of 17 mA may not be efficient enough to readily crosslink the TPU domain dispersed in EVA matrix without the presence of any radiation sensitizer or crosslinking coagent in the blend. However EVA can very easily generate huge number of free radicals and can form a dense crosslinked network structure even at much lower dose range.^9,19 This result has also been supported by the SEM image analysis as discussed earlier. If TPU would have been able to easily crosslink in presence of EVA over this dose range and radiation energy, it would become very difficult to remove the TPU phase by etching the irradiated blends with THF solvent. So, it can be said that under these experimental condition TPU is much less affected by EBR than that of EVA and crosslinked network is predominately formed within EVA matrix.

Fig. 7 Plausible scheme of EVA/TPU blends before and after electron beam radiation.

FTIR spectroscopy

FTIR spectroscopy has been used to characterize the specific changes that might occur in the polymer materials before and after electron beam irradiation in air. Fig. 8a and b show the FTIR spectra of non-irradiated and irradiated (at 50, 100 and 150 kGy radiation doses) ET80 and ET70 blends in the region of 2400–4000 cm⁻¹ respectively. It is observed that in both ET80 and ET70 non-irradiated blends there is a broad intense peak at around 3390 cm⁻¹ corresponding to N–H stretching vibrations of urethane groups of TPU (present in TPU).^41,42 In the IR spectra of electron beam treated ET80 and ET70 blends the intensity of N–H stretching vibration gradually reduces with increasing radiation dose which indicates that there might be intermolecular (within TPU) and interfacial crosslinking to some extent via TPU urethane bond.⁴³ However, it is to be noted that since the extent of this type of crosslink formation via TPU urethane bond is relatively less, the variation in N–H stretching vibration is also small. This is because under this irradiation condition, all the urethane bond can not take part in crosslink formation and this fact has been also supported from SEM analysis. On the other hand there are two strong peaks at 2910 and 2847 cm⁻¹ which can be attributed to symmetric and asymmetric C–H stretching vibrations of –CH₂ groups. It is found that as the radiation dose increases for both the blend system the intensity of C–H stretching vibration slightly decreases may be due to hydrogen abstraction and resulting crosslink formation (more prominent for samples irradiated at 150 kGy).⁶ Fig. 9a and b illustrate the FTIR spectra in the region of 950 to 1300 cm⁻¹. It is also observed that in irradiated blend system, a weak peak appears at 1170 cm⁻¹ which may be due to the formation of crosslinked C–N bond between EVA and TPU as a result of some interfacial interaction after electron beam treatment.⁴⁴

Fig. 8 FTIR spectra for (a) ET80 and (b) ET70 blends before and after irradiation in the range of 2400–4000 cm⁻¹.

Fig. 9 FTIR spectra for (a) ET80 and (b) ET70 blends before and after irradiation in the region of 950–1300 cm⁻¹.

Thermogravimetric analysis

Thermogravimetric analysis has been found to be an effective tool to study the thermal stability of polymer blends. Fig. 10a and b elucidates the effect of radiation dose on thermal degradation behaviour of ET80 and ET70 blends respectively. The thermal analysis data of the samples before and after irradiation are presented in Table 4. It is clearly seen that 5% weight loss temperature (T₉₅), 50% decomposition temperature (T₅₀) and the maximum degradation temperature (T_max) of the irradiated blends gradually shifts to higher temperature with increasing radiation dose. When ET80 and ET70 are irradiated at 100 kGy the 5% decomposition temperature shifts from 323 °C to 337 °C and from 322 °C to 334 °C respectively for the blends. This improvement is due to the formation of more compact crosslinked networks which makes the blend thermally more stable against the formation of gaseous products on heating and postpones the weight loss procedure.^38,45,46 It has also been observed that the maximum decomposition temperature for both the blends at 100 kGy is almost 12–13 °C higher than the unirradiated samples. This result suggests that the thermal stability of the EVA/TPU blend system can be effectively enhanced by the application of electron beam radiation. With further increase of the radiation dose to 200 kGy, the thermal stability of the blends reduces marginally. This slight shift of T₅₀ and T_max to the lower value again indicates that some oxidative degradation and main chain scission have occurred. Moreover, the high concentration of free radicals generated at higher dose rupture the crosslinked structure to some extent and consequently the shorter polymer chain would become easier to decompose.^22,35

Fig. 10 TGA thermograms of (a) ET80 and (b) ET70 blends before and after radiation.

Table 4 TGA and DTG data for the blends before and after irradiation

Sample	Radiation dose (kGy)	T95, temperature corresponding to 5% decomposition (°C)	T50, temperature corresponding to 50% decomposition (°C)	Tmax, maximum decomposition temperature (°C)
ET 80	0	323	447	467
	50	326	451	472
	100	337	457	479
	200	332	457	478
ET 70	0	322	443	466
	50	330	449	472
	100	334	455	479
	200	331	452	476

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Gel content

Gel content of the irradiated blends is shown in Fig. 11 as a function of irradiation dose. It is found that for both the blends gel contents increase with radiation dose and the higher value of gel content suggests the formation of highly crosslinked network structure. The ionizing radiation produced by electron beam accelerator generates extremely reactive species like free radicals and ions which modify the molecular structure of polymeric material by the formation of insoluble chemical crosslinks between molecular chains through radical combination and degradation or chain scission which destroys the molecular structure to some extent. Although both these process occur simultaneously, one plays the major role mainly depending on the chemical structure of the polymer and applied radiation dose.^13,23 For both the EVA/TPU blends, a sharp increase in gel content value was observed as the radiation dose increase from 25 to 150 kGy. As the radiation dose increases from 25 to 150 kGy the gel content value increase from 55.3 and 54.2 to 84.5 and 83.3 for ET80 and ET70 respectively. Further increase in dose to 200 kGy the gel content value changes only marginally. However it has also been observed that at a particular dose gel content value of ET80 blends with higher EVA content has been found to be slightly higher than the ET70 blends. This is because as compared to TPU; EVA can easily form free radicals at lower radiation doses even without the presence of any radiation sensitizer.

Fig. 11 Gel content as a function of radiation dose.

Crosslink density measurement

Swelling measurements were utilized to determine the crosslink density of the samples because at equilibrium the chemical forces tending to dissolve the rubber in solvent are balanced by the restraining forces exerted by the polymer network.⁴⁷ Fig. 12 represents the variation in crosslink density (crosslink density of EVA in presence of TPU) with radiation dose for the blends. It clearly shows that as the radiation dose increases, the overall crosslink density of all the samples continuously increases. On exposure to higher irradiation doses, a large number of radicals are formed, which would lead to denser crosslinked network formation through radical combinations. It is also to be mentioned that ET80 blends exhibit somewhat higher crosslink density than that of ET 70 containing less amount of EVA, because EVA can give instant response towards crosslink formation upon irradiation as compared to TPU.

Fig. 12 Variation of overall crosslink density as a function of radiation dose.

Electrical property

Electrical resistivity study is important for all cable insulation and sheathing material as they must have sufficient ability to restrict the leakage of electrical current. Fig. 13 illustrates the effect of radiation dose on the volume resistance of the EVA/TPU blends. It has been found that for ET80 and ET70 blends volume resistivity increases with electron beam dosage upto 150 kGy but at radiation dose above 150 kGy volume resistivity decreases sharply and a noticeable peak is observed in the plot. However, all the irradiated blends have higher electrical resistance than the blends without irradiation. Since the dose increases from 0 to 150 kGy the volume resistance increases from 3.24 × 10¹⁴ ohm cm to 17.2 × 10¹⁴ ohm cm for ET80 and in case of ET70 the volume resistivity value changes from 2.27 × 10¹⁴ ohm cm to 10.1 × 10¹⁴ ohm cm. Thus the increment in electrical resistivity can be associated with the formation of crosslinked structure induced by electron beam radiation. Hence in irradiated blends the numerous crosslinking points may be considered to act as barrels to prevent the electrical charge movement between polymer chains. The increase in number of traps in a material due to pronounced crosslinking may cause severe restriction in charge mobility and consequently improve the volume resistance of the irradiated samples.^48–50 However, very high radiation dose not only leads to crosslinking but also causes chain scission to some extent. Such molecular chain fragmentation, polar and ionic products due to irradiation^51,52 might be the reason for reduction in electrical resistance at 200 kGy. It is also to be mentioned that generally neat EVA has better electrical resistance than TPU. That is why the improvement in electrical resistance is somewhat less for ET70 blend than the blend containing higher amount of EVA.

Fig. 13 Volume resistivity of the blends with change in radiation dose.

Oil swelling study

Cable sheathing materials are required to have sufficient oil resistance property as they remain exposed to the external environment. The oil resistance property of the samples (before and after radiation) was measured by immersing the samples in IRM 903 oil for 7 days at room temperature. The dependence of oil swelling on radiation dose has been depicted in Fig. 14. The oil resistance property of the EVA/TPU blend system showed remarkable improvement upon irradiation.

Fig. 14 Variation in oil swelling with change of radiation dose.

In our previous paper it has been reported that incorporation of even small amount of TPU significantly increases the oil resistance property of the blends and thus 70/30 EVA/TPU blend was found to show less oil swelling than 80/20 blend.²⁷ Fig. 14 clearly reveals that the oil swell in IRM 903 oil continuously decrease with radiation dose upto 200 kGy. For ET80 oil swell% changes from 40.4 to 17.4, a reduction by 57%, as the radiation dose reaches 200 kGy. Non irradiated ET70 shows almost 36.3% oil swell after 7 days and at 100 kGy the oil swelling becomes only 18.1% and the oil swell further reduces to only 13.8% as the radiation dose increases to 200 kGy which is almost 62% less than the unirradiated ET70 blend. Such a remarkable improvement in oil resistance property can be attributed to the increasing probability of three dimensional network formation and higher degree of crosslink density with radiation dose, which in turn resist the penetration of oil into the blend to cause swelling.

Conclusions

The influence of electron beam irradiation on ET80 and ET70 blends have been analyzed in details and the results reveal that even low radiation doses have significant positive effects on mechanical, thermal and electrical properties of the blend system. Tensile properties of the blends have been found to show dramatic improvement upon irradiation. Tensile strength value of the blends steadily increases with increasing radiation dose upto 150 kGy and then slightly reduces at 200 kGy. Hardness and modulus of all the samples exhibit significant increment with radiation dose with a concomitant decrease in elongation at break. Electron beam induced crosslinked structure improves the interfacial adhesion and imparts stiffness to the blends, which is reflected by the steady increase in storage modulus values upon irradiation. T_g of the blends corresponding to tan δ peak maxima marginally changes after irradiation with a progressive drop in tan δ max peak height which can be ascribed to the hindrance in molecular motion associated with crosslinked structure. Similar variation in T_g values have also been observed from DSC study; however T_m and T_c of the blends corresponding to EVA phase continuously reduces due to lower degree of chain alignment caused by predominant crosslinking which hinders the growth of the crystals. Although the SEM photomicrographs reveal two phase morphology for all the irradiated blends, the gradual reduction in TPU domain size with higher electron dosage suggests better interfacial adhesion and improved compatibility to some extent between the pristine components. FTIR spectroscopy demonstrates the chemical changes in EVA/TPU blends before and after electron beam radiation. Electron beam treatment has also been found to improve the thermal stability of the blends. From electrical test it was observed that the volume resistivity of the blends reaches a maximum at 150 kGy as a result of severe restriction in charge mobility imposed by crosslinking. Gel content and overall crosslink density of the samples continuously increases as a function of irradiation dose. There was a remarkable improvement in oil resistance property after irradiation and the improvement is more prominent for higher TPU containing (EVA/TPU 70/30) blend. Overall, EVA/TPU 70/30 blend at 100 kGy has been found to be a very good cost effective material with optimum combination of mechanical, thermal, electrical and oil resistance property suitable for cable sheath application.

Acknowledgements

The authors would like to thank Dr Swati Neogi (IIT Kharagpur, India) and Mr Debdipta Basu (IPF Dresden, Germany) for their kind help in thermal characterization.

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