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Effect of Trimethylolpropane Triacrylate (TMPTA) on the Properties of Irradiated Epoxidized Natural Rubber (ENR-50), Ethylene-(vinyl acetate) Copolymer (EVA), and an ENR-50/EVA Blend

M. Zurina,1 H. Ismail,2 C.T. Ratnam3 1 Department of Polymer Engineering, Faculty of Chemical and Natural Resource Engineering, University Technology Malaysia, 81310 Skudai, Johor, Malaysia 2

Division of Polymer, School of Materials and Mineral Resources Engineering, University Sains Malaysia, Seberang Perai Selatan, Pulau Pinang, Malaysia

3

Malaysian Institute for Nuclear Technology Research (MINT), Bangi, 43000 Kajang, Malaysia

The effect of trimethylolpropane triacrylate (TMPTA) monomer on the tensile properties, dynamic mechanical properties, and morphology of irradiated epoxidized natural rubber (ENR-50), ethylene-(vinyl acetate) copolymer (EVA), and an ENR-50/EVA blend was investigated. The ENR-50, EVA, and ENR-50/EVA blend were irradiated by using a 3.0-MeV electron-beam apparatus at doses ranging from 20 to 100 kGy. The improvement of tensile properties and morphology with irradiation indicated the advantage of having irradiation-induced crosslinks in these materials. Observation of the properties studied confirmed that TMPTA was efficient in enhancing the irradiation-induced crosslinking of ENR50, EVA, and the ENR-50/EVA blend. Addition of TMPTA improved the adhesion between the ENR-50/EVA blend phases by forcing grafting and crosslinking at a higher irradiation dose (100 kGy). J. VINYL ADDIT. TECHNOL., 15:47–53, 2009. ª 2009 Society of Plastics Engineers

INTRODUCTION Blending of polymers is a common technology, that is frequently applied to develop a product with superior mechanical properties from inexpensive polymeric materials and small amounts of compatibilizers. The aim of making polymer blends, i.e., mixtures of two or more polymers, is to obtain materials which, as far as possible, combine the advantages of their components [1, 2]. Electron-beam (EB) irradiation has certain advantages over conventional processes, such as the absence of any catalyst residue (because the process does not require catCorrespondence to: M. Zurina; e-mail: [email protected] DOI 10.1002/vnl.20175 Published online in Wiley InterScience (www.interscience.wiley.com). Ó 2009 Society of Plastics Engineers

JOURNAL OF VINYL & ADDITIVE TECHNOLOGY— —2009

alysts to initiate crosslinking) and the complete control of temperature. It is also solvent-free and a source of enormous amounts of radicals and ions. Irradiation crosslinking takes place at room temperature; thus it is suitable for thermally sensitive components which may degrade upon heat treatment. Besides this, the dosage of irradiation can be controlled easily. As regards radiation effects on polymers, it is wellknown that the main polymer chains can be either degraded or crosslinked by ionizing irradiation, processes which can occur concurrently. Therefore, to maximize the radiation-induced crosslinking of polymers, it is useful to add multifunctional monomers as crosslinking agents [3]. In this work, an attempt was made to study the effect of trimethylolpropane triacrylate (TMPTA), on the properties of epoxidized natural rubber (ENR-50), ethylene-(vinyl acetate) copolymer (EVA), and an ENR-50/EVA blend after EB irradiation. The ENR is a chemically modified natural rubber (NR). The epoxidation of NR to produce ENR involves the random creation of epoxide groups from the double bonds of the NR polymer chains. The ENR possesses excellent properties, such as oil resistance, gas impermeability, good wet grip, and high damping characteristics [4]. The oil resistance of ENR-50 is due to the polarity of the epoxide group, and the hydrocarbon resistance of NR is found to be improved with an increase in epoxidation level. However, the resistance to polar solvents shows a decline with increasing epoxide content in NR [5]. Currently, the Malaysian Rubber Board produces ENR with the trade name Epoxyprene. Two grades are available, namely, ENR-25 and ENR-50, with 25 and 50 mol% of epoxidation, respectively. The market applications for ENR were found to be limited. Thus, attempts are being

made to diversify the usage and applications of this rubber, especially in advanced engineering fields. As mentioned above, blending with other polymers is the easiest and cheapest way to tailor the properties of ENR and at the same time reduce the material cost. The EVA copolymers are randomly structured polymers which offer excellent ozone and weather resistance and have remarkable mechanical properties [6]. They are also halogen-free thermoplastics. Therefore, an EVA was chosen to be blended with ENR-50, and it was hoped that the blend would lead to the production of halogen-free materials that would suit many applications which are currently dominated by plasticized PVC. This study investigated several characteristics of the sensitizing effect of TMPTA on the radiation-induced crosslinking of ENR-50, EVA, and an ENR-50/EVA blend. The properties of the blend materials were examined in several ways that included the determination of gel content, tensile properties measurements, dynamic mechanical analysis (DMA), thermal gravimetric analysis (TGA), and morphology inspection by scanning electron microscopy (SEM). EXPERIMENTAL Materials Epoxidized natural rubber ENR-50 with 50 mol% of epoxidation (Grade EPOXYPRENE 50) was obtained from the Malaysian Rubber Board, with a specific gravity value of 1.03. Ethylene–(vinyl acetate) copolymer (Grade H2020) having 15 mol% of vinyl acetate, an MFI value of 1.5 g/10 min, and a density of 0.93 g/cm3 was purchased from The Polyolefin Company, Singapore. Trimethylolpropane triacrylate was a product of UCB Asia Pacific, Malaysia and was used as received. Its structure is shown in Table 1. Mixing Procedure The ENR-50, EVA, and ENR-50/EVA blend (50/50) were prepared by melt-mixing in a Haake Rheomix Polydrive R 600/610 apparatus at 1208C and a rotor speed of 50 rpm. The EVA was charged into the mixing chamber and allowed to melt for 2 min. Then ENR-50 and TMPA [4 phr (parts by weight per hundred parts of resin)] were added to the molten EVA, and the mixing was continued for a further 4 min. For pure EVA of ENR-50, the material was blended with TMPTA (4 phr) for 6 min. The samples were then compression-molded at 1208C for 5 min. Afterwards, the samples were cooled for 2 min to produce 1- and 2-mm thick sheets.

TABLE 1. Structure of crosslinking agent (TMPTA). Name

Molecular structure

Trimethylolpropane triacrylate (TMPTA)

range of 0–100 kGy. The acceleration energy, beam current, and dose rate were 2 MeV, 2 mA, and 20 kGy per pass, respectively. Gel Content The gel content of the crosslinked samples was determined by extraction of the samples with boiling xylene for 24 h in a Soxhlet apparatus. The extracted samples were dried in an oven at 508C to constant weight, and the gel content was calculated as follows: % Gel content ¼

W0 W1 3 100 W0

where W1 and W0 are the weight of the dried sample removed by extraction and the weight of the sample before extraction, respectively. Tensile Properties The tensile properties were measured with an M 500 tensometer according to ASTM D 412 at a 50 mm/min crosshead speed. The molded samples of 1-mm thickness were cut into standard test pieces by using a Wallace die cutter. Five samples were used for the tensile test, and an average was taken as the resultant value. Dynamic Mechanical Analysis Dynamic mechanical properties were measured with a Perkin-Elmer dynamic mechanical analyzer, Mode 1 DMA 7e. The experiment was conducted in a three-point bending mode at a frequency of 1 Hz. The temperature was increased at 58C/min over the range of 270 to 308C. The samples were 2-mm thick, 15-mm long, and 10mm wide. Morphology Study

Irradiation The molded sheets were irradiated by using a 3-MeV electron-beam accelerator (NHV EPS-3000) at a dose

48 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY— —2009

Examination of cryogenically fractured surfaces etched with nitric acid was performed with a scanning electron microscope Model Leica Cambridge S-360. All samples

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FIG. 1. Gel content of the ENR-50/EVA blend and its components at different irradiation doses.

were examined after sputter-coating with gold to avoid electrostatic charging and poor image resolution. RESULTS AND DISCUSSION Gel Content Figure 1 shows the trends in the gel that is formed in control and TMPTA-containing samples. Generally, the extent of the irradiation-induced crosslinking (crosslink density) of polymers can be estimated from gel content determination. A higher value of gel content indicates a more extensive crosslinking reaction. All the samples show a sharp increase in the percentage of gel at the irradiation dose of 20 kGy. The samples containing TMPTA show larger increments in the percentage of gel than the control samples. This difference indicates that TMPTA can enhance the crosslinking reaction of the irradiated EVA, ENR-50, and ENR-50/EVA blend. That conclusion is supported by the increased Tg values of the irradiated samples containing TMPTA (Table 2).

Tensile Properties Tensile Strength. The tensile properties are functions of crosslink density, molecular imperfection, molecular weight reduction due to degradation, etc. Figures 2–4 show the influence of TMPTA on the tensile strength of EVA, ENR-50, and the ENR-50/EVA blend at different irradiation doses. It can be seen in Fig. 2 that the tensile strength of the control EVA increases with increases in

FIG. 2. Effect of TMPTA on the tensile strength of pure EVA at different irradiation doses.

irradiation dose, whereas that of EVA containing TMPTA increases at 20 kGy and then decreases with further increases in dose. This difference was due to the chain scission and excessive crosslinking occurring with TMPTA at doses of 40 kGy and above, such that the tensile strength was reduced. This conclusion is supported by the DMA data in a later section. The optimum gel content induced by TMPTA was 64% at 20 kGy. Meanwhile, the optimum gel content induced in the control sample was 75% at 60 kGy. It is observed that TMPTA is able to reduce the irradiation dose required to achieve the optimum tensile strength. In the case of ENR-50, Fig. 3 shows the tensile strength at different irradiation doses. The tensile strength increases with an increase in irradiation dose for all the samples. It is observed that ENR-50 containing TMPTA reaches a higher tensile strength at lower dosage in comparison with the behavior of control samples. In the ENR-50/EVA blend (see Fig. 4), the tensile strength increases with increasing irradiation dose for all the samples. There is a sharp increase at 0–40 kGy and a slight increase thereafter for the samples with the crosslinking agent. These results are in accordance with the gel content data. During the initial irradiation, a large number of radicals is generated in the TMPTA molecules. As a result, they undergo extensive crosslinking and grafting reactions with ENR-50 and EVA that cause drastic changes in tensile properties. However, the consumption of the acrylate during the initial stage reduces the efficiency of irradiation-induced crosslinking at higher doses,

TABLE 2. Tg values (8C) of the ENR-50/EVA blend and its components at different irradiation doses. ENR-50/EVA (50:50)

EVA

ENR-50

Samples Dose (kGy)

Control

TMPTA

Control

TMPTA

Control

TMPTA

0 60 100

26 11 14

3.5 22.4 23.0

215 211.5 213

27.5 27 25.4

213.5 27.5 210.2

214.4 28.0 26.3

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FIG. 3. Effect of TMPTA on the tensile strength of pure ENR-50 at different irradiation doses.

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FIG. 4. Effect of TMPTA on the tensile strength of the ENR-50/EVA blend at different irradiation doses.

thus accounting for the marginal change in tensile strength at doses of 60 kGy and above. It is also important to note that at any irradiation dose, the ENR-50/EVA blend with TMPTA registered a higher tensile strength than to the control blend. This result could be attributed to the increased formation of crosslinks in the samples with TMPTA, as is evident from gel content analysis. It could be noted that, without irradiation, TMPTA acts as a plasticizer that imparts lubrication to the EVA and the ENR-50/EVA blend, thus reducing their tensile strength. Similar findings were also reported by Patacza et al. [7].

FIG. 6. Tan delta as a function of temperature for EVA with and without TMPTA at different irradiation doses.

The position and height of the tan d peak in the dynamic mechanical spectrum of a polymer are indicative of the degree to which a polymer is crosslinked or modified [9]. The temperature at the tan d peak maximum is taken as the glass transition temperature (Tg) of the sample.

Figure 6 shows tan delta for EVA as a function of temperature at different irradiation doses. The Tg of the unirradiated control EVA is found to be 268C. This Tg increases with increases of irradiation dose (Table 2). The Tg values are further increased by the introduction of TMPTA. These phenomena are in accordance with the increasing degree of modification confirmed by crosslink density (gel content), which registered an increase with increases in irradiation dose and with the introduction of crosslinking agent. There was no gel found in unirradiated EVA with and without TMPTA, a result which indicated that no crosslinks were present. We believe that the shift in Tg values and the slight reduction in tan delta maximum (tan dmax) values with the introduction of TMPTA were due to some degree of interaction between EVA and TMPTA during mixing. The tan dmax value shows the ability of the material to flow. It is expected that increases in irradiation dose will increase the Tg and thus reduce the tan dmax values, owing to the existence of irradiationinduced crosslinks which restrict the mobility of the molecules. However, at 60 kGy the tan dmax values of EVA with TMPTA increase. This increase was due to the excessive radical formation at this dosage compared with crosslinking, which a situation facilitates the easy movement of EVA molecules. Figure 7 shows the storage modulus as a function of temperature for EVA at different irradiation doses. As can be seen, the storage modulus in the transition region

FIG. 5. Effect of TMPTA on the elongation at break of pure EVA, the ENR-50/EVA blend, and pure ENR-50 at different irradiation doses.

FIG. 7. Storage modulus as a function of temperature for EVA with and without TMPTA at different irradiation doses.

Elongation at Break. In general, the elongation at break (see Fig. 5) follows the sequence of control [ TMPTA. The results are in direct contrast to the trend observed for percentage of gel. Papiya and Bhowmick [8] explained that elongation at break is reduced with increasing crosslink density and limited extensibility in the initial stage and increased because of chain scission at higher irradiation doses.

Dynamic Mechanical Analysis


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FIG. 8. Tan delta as a function of temperature for ENR-50 with and without TMPTA at different irradiation doses.

FIG. 10. Tan delta as a function of temperature for the ENR-50/EVA blend with and without TMPTA at different irradiation doses.

increases with increases in irradiation dose, and the TMPTA further enhances the storage modulus. The storage modulus values are indicative of the stiffness of the materials. Increases in irradiation-induced crosslinking will increase the stiffness of the EVA and thus increase the storage modulus. Unirradiated EVA with TMPTA registered a lower storage modulus than the control. This difference was due to the lubrication action imparted by the crosslinking agent to the EVA. These results agree with the findings for tensile strength. In the case of ENR-50, unirradiated ENR-50 with TMPTA shows a slight decline in Tg (Fig. 8 and Table 2). This decrease might be due to the lubrication action of the crosslinking agent. At 60 kGy, the existence of irradiation-induced crosslinking caused the reduction in tan dmax values. On the other hand, the excessive radical formation at this stage causes marginal changes in Tg. At higher irradiation doses, TMPTA enhances the crosslink formation and thus increases the Tg of the samples. Figure 9 shows the storage modulus as a function of temperature for ENR-50 at different irradiation doses. As can be seen, the storage modulus at the transition region of unirradiated ENR-containing 50 TMPTA is slightly reduced because of the lubrication action that reduces the stiffness of the ENR-50. At 60 kGy, the same trend is observed, owing to the excessive radical formation in

TMPTA. At a higher irradiation dose, TMPTA has enhanced the formation of irradiation-induced crosslinks that increase the stiffness of the sample. In the ENR-50/EVA blend (see Fig. 10), TMPTA has increased the Tg value of the unirradiated material. This increase may be due to good interaction between TMPTA and the blend constituents. At 60 kGy, the TMPTA enhances the crosslink formation, thus increasing the Tg of the blend. At 100 kGy, the blend has become brittle because of excessive crosslinking, a conclusion that is supported by the storage modulus values (Fig. 11).

FIG. 9. Storage modulus as a function of temperature for ENR-50 with and without TMPTA at different irradiation doses.

FIG. 11. Storage modulus as a function of temperature for the ENR50/EVA blend with and without TMPTA at different irradiation doses.

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MORPHOLOGY The morphology of cryogenically fractured and selectively extracted ENR-50 phase surfaces of the ENR-50/ EVA blend (50/50 blend ratio) can be evaluated from Fig. 12. As can be seen, the pure ENR-50/EVA blend (Fig. 12A) demonstrates a cocontinuous morphology. The unirradiated blend with TMPTA shows a smooth and soft morphology where TMPTA acts as a diluent. As irradiation dose increases (60 kGy), the ENR-50 and EVA start forming a cocontinuous morphology. Upon irradiation, grafting and crosslinking take place, likely more in one phase than in the other. At a higher irradiation dose (100 kGy), the blend with TMPTA shows a


FIG. 12. SEM micrographs of the ENR-50/EVA blend with and without crosslinking agents at different irradiation doses (32000).

compatible morphology where only one phase can be seen. The ENR-50 and EVA are mostly crosslinked to each other, thus producing the one-phase morphology. This result shows that TMPTA is effective in the crosslinking reaction and enhances the compatibility of the irradiated ENR-50/EVA blend. This conclusion also is supported by the existence of one tan delta peak and one Tg value for the ENR-50/EVA blend at this dosage.

significantly enhanced the irradiation-induced crosslinking of all of these materials. The existence of irradiationinduced crosslinks led to improvements in the tensile properties and morphology of the samples containing TMPTA. The ENR-50/EVA blend remained compatible when in the crosslinked condition.

REFERENCES CONCLUSIONS Irradiation-induced crosslinking occurred in ENR-50, EVA, and an ENR-50/EVA blend. Addition of TMPTA


1. S.K. De and A.K. Bhowmick, Thermoplastic Elastomers from Rubber-Plastic Blend, Ellis Harwood, New York (1990).

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2. M. Zurina, H. Ismail, and C.T. Ratnam, J. Appl. Polym. Sci., 99, 1504 (2006). 3. I.J. Lee, H.W. Choi, Y.C. Nho, and D.H. Suh, J. Appl. Polym. Sci., 88, 2339 (2003). 4. I.R. Gelling, J. Nat. Rubber Res., 6, 184 (1991). 5. C.S.L. Baker and I.R. Gelling, ‘‘Epoxidised Natural Rubber’’ in Development in Rubber Technology, Vol. 4, A. Whelan and K.S. Lee, Eds., Elsevier Applied Science, London, 87 (1987).

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6. K.W. Doak, ‘‘Ethylene Polymer’’, in Encyclopedia of Polymer Science and Engineering, Vol. 6, H.F. Mark, N.M. Bikales, C.G. Overbeger, and G. Menges, Eds., Wiley, New York, 383 (1986). 7. C. Patacza, X. Coquereta, and C. Decker, Radiat. Phys. Chem., 62, 403 (2001). 8. S.M. Papiya and A.K. Bhowmick, J. Appl. Polym. Sci., 77, 323 (2000). 9. C.T. Ratnam and K. Zaman, Polym. Degrad. Stab., 65, 481 (1999).
