Effect of electron beam irradiation on the properties of EVA/EPDM blends

Abstract

Effect of electron beam (EB) irradiation on the properties of ethylene vinyl acetate (EVA)/ethylene–propylene–diene monomer (EPDM) (50/50) blends was studied. The blends were firstly melt-compounded at 130°C followed with being irradiated using 4.0 MeV EB energy at doses ranging from 0 kGy to 200 kGy. It is found that the dosage of irradiation plays a key role in the properties of the blends. With the increasing dosage of irradiation, tensile strength and thermal stability were enhanced. The irradiation exerts a cross-linking effect on the blends, and the increase in density is responsible for the enhanced properties. Dynamic mechanical and thermal analyses and morphology indicate that irradiation does not play any negative role in the compatibility between EVA and EPDM. Hot set test reveals that irradiation could improve the heat deformation property of blends. Thus, it is reasonable and interesting to modify EVA/EPDM blends using EB irradiation to further increase its properties.

Keywords

EVA EPDM electron beam irradiation cross-linking heat deformation property

Introduction

In recent years, electron beam (EB) irradiation is a popular method for the modification of polymer properties.^1

-7 Cross-linking induced by silane coupling agents, peroxide, or irradiation can enhance the flammability and thermal and mechanical properties of the polymers. Comparing with the silane cross-linking or peroxide cross-linking process, EB irradiation has several advantages, such as efficient process at room temperature, pollution-free before or after the irradiation, and easy implement for production.^8
-10 Therefore, EB irradiation has been widely used as a method for the modification of polymers, especially for polyolefin.

Thermoplastic/elastomer materials have lots of important applications including wire and cable industrial areas. Ethylene vinyl acetate (EVA) copolymers are randomly structured polymers that have excellent weather resistance, ozone resistance, and mechanical properties. It is widely applied for making heat shrinkable tubes, cable insulation coats, and flame retardant insulation^11

-15 based on these benefits. On the other hand, ethylene–propylene–diene monomer (EPDM) has high thermal resistance, suitable lifetime, resistance against many chemicals, and good insulating properties. Mixing two different polymers together makes it possible to achieve various property combinations of the final material. Polymer blending is usually a more economically viable method than synthesizing new polymers. Therefore, we focus on the blending of EVA/EPDM to combine different properties of each component. EB irradiation cross-linking of polymer blends involves the formation of three-dimensional structures causing substantial changes in material properties and it can enhance its mechanical properties and thermal stabilities.^16

-19 In addition, the polyolefin cross-linking formed by the EB irradiation method also gives rise to better properties, such as weather resistance and ozone resistance. Therefore, modification of polyolefin blends using EB irradiation is an attractive method for the development of materials in wire and cable industry or other specialty applications.

Actually, many studies on EB modification of EVA have been reported. For example, Matsui et al.²⁰ have studied the changes in mechanical properties of EVA film irradiated by EB. They figured out that the optimal dose is 200 kGy required for getting the maximum percent gel of 50 g kg⁻¹ EVA film. In another study,²¹ mechanical, thermal, and electric properties of EVA by EB irradiation were investigated. This study found that EB-irradiated EVA is more thermally and mechanically stable than pure EVA. The electrical properties revealed that surface and volume resistance were enhanced. In addition, a series of studies on EB irradiation cross-linking of EVA blended with polymers like Linear low-density polyethylene (LLDPE), Low density polyethylene (LDPE), High Density Polyethylene (HDPE) or Polypropylene (PP) are also reported.^22

-29 With regard to EVA/EPDM blends, Mishra et al.³⁰ have studied thermal stability of different fraction of EVA in EVA/EPDM blends. They found that EVA/EPDM at 80/20 blends composition exhibits the maximum thermal stability under nitrogen (N₂) atmosphere. According to the previous study,³¹ the mechanical, thermal, and electric properties of EPDM/EVA blends as cable insulation under aging condition were reported. It is found that EVA/EPDM blends are suitable for making materials in wire and cable insulation. At the same time, Chowdhury et al.³² and Sharma et al.³³ found that the content of vinyl acetate (VA) in EVA will affect the compatibility and mechanical properties of the EVA/EPDM blend (different contents of EVA). However, according to our knowledge, the properties of EVA/EPDM blend before or after being irradiated have not been reported in these studies. The modification using EB irradiation of such polyolefin is widely used as materials for heat shrinkable tubes and wire or cable insulation.^34,35 Thus, it is reasonable and interesting to modify EVA/EPDM blends using EB irradiation to further increase its properties.

In this work, EVA is blended with EPDM, and triallyl isocyanurate (TAIC) works as a multifunctional monomer to enhance the irradiation cross-linking properties of the blend. The author focuses on the properties of EVA/EPDM blends with the treatment of EB irradiation at different doses ranging from 0 kGy to 200 kGy. Hence, the current work reports on the effects of EB irradiation on physical, mechanical, and thermal properties of EVA/EPDM blends. Mechanical tests and hot set tests were carried out. Dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) were used to investigate the thermal properties of unirradiated and irradiated samples. Scanning electron microscopy (SEM) is utilized to investigate the fractured morphology.

Experiment

Materials

The EVA (260 A, Melt mass-flow rate (MFR) = 6 g/10 min, 28 wt% VA content) was supplied by DuPont, USA, and EPDM (4770 R,70 wt% ethylene content, 25 wt% propylene content, and 5 wt% 5-ethylidene-2-norbornene content) was provided by Dow Chemical Company, Wilmington, Delaware, USA. The irradiation sensitizer, TAIC, was supplied by Jiangsu Huaxing Chemical Co. Ltd, China.

Preparation of samples and irradiation

The EVA/EPDM (50/50) blend with 3 phr TAIC was prepared via melt compounding at 130°C for 8 min in a Thermo Haake Rheo mixer (R600, Thermo Electron Corporation, Germany) with a rotation speed of 60 r min⁻¹. Then, the compounds were compression molded in a mold (100 × 100 × 1 mm³) under the condition of 130°C for 10 min and 10 MPa pressure. Irradiation of the sample sheets was carried out using an electron accelerator (Wuxi Elpont Radiation Technology Co. Ltd, China) with energy of 4 MeV and 10 mA. Various irradiation doses (0–200 kGy) were used, and the irradiation was carried out in air at room temperature.

Testing and characterizations

Mechanical properties

Tensile strength and elongation at break were measured with a universal testing machine (Instron 3367, Instron, USA) at a crosshead speed of 200 mm min⁻¹ according to ISO 527-1-2012.

Measurement of cross-linking density

The cross-linking density (XLD model) measurements were done using Micro MR-CL (Shanghai Niumag Corporation Ltd, China) cross-linking density spectrometer. And the testing temperature for EVA/EPDM samples was 90°C. The XLD model equation is expressed as follows:

M (t) = A * exp (- \frac{t}{T_{21}} - \frac{1}{2} q M_{r l} * t^{2}) + B * exp (- \frac{t}{T_{21}}) + A_{0}

where M(t) is the detected signal corresponding to the magnetization decay function, A is the amplitude of the rigid fraction of network, T ₂₁ represents to the spin–spin relaxation time, qM_rl is the remaining dipolar magnetic coupling of the hydrocarbon chain protons as a result of the anisotropic motion of the chain segments, B is the relative amount of the mobile fraction of polymer, and A ₀ is the compensation for a possible offset of the signal.

Scanning electron microscopy

The SEM micrographs of tensile-fractured surfaces for the tensile samples were explored by an S-4800 field-emission scanning electron microscope (Hitachi, Japan) with 15-kV accelerating voltage.

Thermal characterizations

DMA (DMA242c, Netzsch, Germany) was performed to determine the dynamic mechanical properties. The dimensions of the specimens for tests were 20 × 6 × 1 mm³. All tests were carried out in extension mode at a frequency of 1 Hz and a heating rate of 3°C min⁻¹ with a temperature range of −80°C to 80°C.

TGA was done in a TGA simultaneous thermal analyzer (409PC, Netzsch, Germany) at a scanning rate of 10°C min⁻¹ under N₂ from 20°C to 800°C.

The hot set test was finished in a hot set oven. All the samples with the dumbbell shape were placed in the hot set oven under a definite static load at 200°C and the elongation between two marks was measured after 15 min.

Results and discussion

Figure 1 shows the influence of irradiation dose on the mechanical properties of EVA/EPDM (50/50) blends. The tensile strength increases as the irradiation dose increases. For example, as the irradiation dose increases from 0 kGy to 100 kGy, the tensile strength increases from 23.2 MPa for control sample to 25.3 MPa. However, such an increasing trend becomes less as the irradiation dose is more than 100 kGy. This phenomenon could be explained by the chain scission at a high dose.³⁶ Although the increase in tensile strength is observed, the loss in elongation at break is accompanied at the same time. The elongation at break almost exhibits a linear decrease with the increasing irradiation dose. This result is expected, since more cross-links could be produced as the irradiation dose increases, which prevents the reorganizations of polymeric chains under the condition of drawing.

Figure 1.

Mechanical properties of EVA/EPDM (50/50) blends with different irradiation doses.

Effects of different radiation doses on cross-link density of EVA/EPDM composites

Many investigations^37

-41 on rubbers have been made using solid-state Nuclear Magnetic Resonance (NMR) spectroscopy. Some researchers⁴² use XLD molder for practical application. In our study, we calculated the cross-link density of the samples at 90°C using solid-state NMR spectroscopy.

Figure 2 shows the influence of irradiation doses on the cross-link densities of the EB-irradiated samples with 3 phr TAIC loading. It is noted that the cross-link density of EB-irradiated samples increased as we increased the irradiation dose. However, at the irradiation dose ranging from 100 kGy to 200 kGy, the cross-link density increased slightly. It is well-known that under the irradiating condition, cross-link and degradation compete with each other.⁴³ Perhaps, under the condition of high-energy irradiation, degradation also takes part in the mechanism, and the formed network could be broken down and the formation of transvinylene could be easier.⁴⁴ It should be pointed out that the trend of cross-link density correlates well with the tendency of tensile strength. This phenomenon indicates that the irradiation should be responsible for the improvement in tensile strength. On the other hand, both the tensile strength and cross-link density slightly increase as the irradiation dose increases from 100 kGy to 200 kGy. This reveals that 50 kGy is sufficient enough to increase the tensile strength, and further irradiating energy (above 50 kGy) would be of less significance.

Figure 2.

Cross-link density of EVA/EPDM (50/50) blends with different irradiation doses.

DMA on EVA/EPDM composites at different radiation doses

DMA was used to determine the transition behavior of neat EVA/EPDM blends and the irradiated blends. The temperature dependence tan δ of EVA/EPDM blends with different irradiation doses is shown in Figure 3. It is clearly seen from Figure 3 that the irradiation doses play a key role in the transition behaviors of blends. For neat blends, only one peak is observed at about −24.5°C, indicating the good compatibility between EPDM and EVA. Acharya et al.⁴⁵ reported that EPDM has a β transition at about −60°C, while EVA exhibits a β transition at about −36°C. Their blends exhibit partial compatibility. However, in our work, only one peak is observed. Perhaps, this is determined by the nature of EPDM and EVA used in this article. With the increasing doses of irradiation, one can see that only one peak. Thus, it could be concluded that irradiation plays no negative role in the compatibility between EVA and EPDM.

Figure 3.

Variation curves of loss factor (tan δ) of EVA/EPDM (50/50) in different radiation doses with temperature.

However, as doses of irradiation increases to 50 kGy, one can see that the intensity of peak decreases. This indicates that fewer polymer chains participate in this transition. The decrease in intensity together with the lower peak temperature value can be attributed to the cross-link structure caused by irradiation. However, when the dose of irradiation further increases to 150 kGy, it is interesting to note that both the intensity and temperature at peak value are higher compared with samples that are irradiated under the condition of 50 kGy. This feature might be explained by the possibility that some macromolecular chains are degraded and some formed networks could be broken down, based on which to the transition behavior could be strengthened. However, further investigation is required and is being carried out to verify this explanation.

SEM on EVA/EPDM composites at different radiation doses

SEM is used to reveal the phase morphology of EVA/EPDM blends and the influence of irradiation on the phase morphology of EVA/EPDM blends. As can be seen in Figure 4(a), no obvious phase separation or any two-phase structure is observed, verifying the good compatibility between EVA and EPDM. After irradiation, one could not observe any obvious different characters of morphology (see Figure 4(b) and (c)), that is, a homogeneous phase structure is still retained. This indicates that the irradiation does not exert any negative effect on the compatibility between different components in binary blends and is consistent with the results of DMA.

Figure 4.

Fracture morphology of EVA/EPDM (50/50) blends with different irradiation doses. (a) 0 kGy, (b) 50 kGy, and (c) 150 kGy.

TGA on EVA/EPDM composites at different radiation doses

TGA is used to evaluate the thermal degradation and thermal stability of neat blends and blends that are irradiated. Pure EVA exhibits a two-degradation step: the first corresponds to the decomposition of VA from EVA molecule and the second is related to the crack of the polyethylene formed in the first step,²¹ while pure EPDM is degraded within the only step. Thus, it can be expected that pure EVA/EPDM blends exhibit a two-step degradation behavior (see Figure 5(a)). As the irradiation dose increases, EVA/EPDM blends still exhibit a two-degradation step, indicating that irradiation does not play any substantial role in thermal degradation.

Figure 5.

(a) TG and (b) DTG curves of EVA/EPDM (50/50) blend.

Although the influence of irradiation dose in degradation step could be neglected, the irradiation seems to enhance the thermal stability of pure blends. As presented in Table 1, T ₁₀ is enhanced as the irradiation dose increases. For example, as the irradiation dose increases to 150 kGy, T ₁₀ is enhanced from 415°C to 425°C, indicating the improvement in thermal stability. It should be pointed out that the improvement in T ₁₀ is not combined with the increase in T ₅₀. T ₅₀ almost remains constant with the increasing irradiation dose. This interesting phenomenon suggests that the irradiation does not exert any effect in the second-degradation step of blends (since T ₅₀ is located in the temperature region of the second-degradation step). In contrast, due to the improvement in T ₁₀, it plays a key role in the first-degradation step (the deacetylation reaction of EVA).

Table 1.

STA data for the different irradiation samples.

	TGA data (°C)			Melting point (°C)
	T ₁₀ ^a	T ₅₀ ^b	T _max ^c	Melting point (°C)
0 kGy	415	467	468	89.2
150 kGy	425	469	474	85.8

STA: simultaneous thermal analyzer.

^a10% Weight loss temperature.

^b50% Weight loss temperature.

^c Temperature of maximum thermal degradation.

It is well accepted that the elimination of an α-hydrogen is required for deacetylation reaction and formation of ketone, since the α-hydrogen is more liable due to the strong −I effect caused by ester groups.⁴⁶ For the same reason, free radicals are preferred to be gained on the α-carbon atoms in EVA during irradiation, which in turn helps to form the cross-link structure. Simultaneously, this decreases the number of decomposition site, and thus, the deacetylation is hindered, based on which the thermal stability is improved.

The influence of irradiation on EVA is also verified by the change in the melting point of EVA. As presented in Table 1, the melting point of EVA decreases as the dose of irradiation increases. We can infer that the crystalline regions may become slightly impaired due to cross-link structure formed in the EVA/EPDM blends.

Hot set test on EVA/EPDM composites at different radiation doses

The results of hot set tests are summarized in Table 2. The neat EVA/EPDM blends fail immediately in the hot set test. However, the irradiation dose plays a positive role in the hot set test: only with the irradiation dose of 50 kGy is sufficient enough to guarantee the samples that do not fail immediately. As the irradiation dose increases from 50 kGy to 200 kGy, the elongation decreases to 45%, indicating the enhanced heat deformation property. This is expected and is due to the increase in the cross-link density and the formation of network structure, which restricts the melt flow of the heated polymeric samples.¹⁸

Table 2.

Hot set results at 200°C under 20 N cm⁻² load for 15 min.

Irradiation dose (kGy)	Elongation
0	Fails immediately
50	>250%
100	90%
150	55%
200	45%

Conclusion

The effect of EB on mechanical and thermal properties and compatibility of EVA/EPDM (50/50) blends is investigated in this study. The results reveal the positive role played by irradiation on the tensile strength and thermal properties of EVA/EPDM blends. Mechanical tests reveal that the tensile strength of neat samples is enhanced after being irradiated, though this enhancement is combined with the decrease in elongation at break. TGA tests indicate that irradiation enhances the thermal stability of blends, and such improvement is possibly due to the hindering effect of irradiation on the deacetylation reaction of EVA. Both the results of DMA and SEM indicate that irradiation exerts no negative role in the compatibility between EVA and EPDM. The hot set tests reveals that the improved heat deformation properties after being irradiated are mainly due to the increasing cross-link density caused by irradiation.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Haoyang Chen

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