Protective effects in radiation modification of elastomers

Radiation Physics and Chemistry 105 (2014) 53–56

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Protective effects in radiation modification of elastomers Wojciech Głuszewski a,n, Zbigniew P. Zagórski a, Maria Rajkiewicz b a b

Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland Institute for Engineering of Polymer Materials and Dyes, Harcerska 30, 05-820 Piastów, Poland

H I G H L I G H T S








We described the processes of radiation/peroxide crosslinking of the elastomer. Aromatic peroxides have a protective effect in the radiolysis of elastomers. Aromatic additives have a significant impact on crosslinking and oxidation. It is possible synergy of radiation and peroxide crosslinking. To study the aging process elastomers used the method of DRS.

art ic l e i nf o

a b s t r a c t

Article history: Received 5 February 2014 Accepted 29 June 2014 Available online 14 July 2014

Saturated character of ethylene/octene thermoplastic elastomers demands an application of nonconventional methods of crosslinking connections between chains of molecules. These are organic peroxides, usually in the presence of coagents or an application of ionizing radiation. Several approaches (radiation, peroxide, peroxide/plus radiation and radiation/plus peroxide) were applied in crosslinking of elastomere Engage 8200. Attention was directed to the protection effects by aromatic peroxides and by photoand thermostabilizers on radiolysis of elastomers. Role of dose of radiation, dose rate of radiation as well as the role of composition of elastomere on the radiation yield of hydrogen and absorbtion of oxygen was investigated. DRS method was used to follow postirradiation degradation. Influence of crosslinking methods on properties of elastomers is described. Results were interpreted from the point of view of protective actions of aromatic compounds. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Elastomer Protective effect Crosslinking, ethylene/octane rubbers

1. Introduction Thermoplastic elastomers, as saturated polymers with a low number of double bonds are characterized by high resistance to ozone and thermal aging, but exhibit unsatisfactory mechanical properties. It is believed that the peroxide–radiation crosslinked thermoplastic elastomers achieve better properties than those crosslinked by conventional methods. The undeniable advantages of radiation techniques include the possibility of crosslinking of the elastomer at ambient temperature and are relatively easy to control by the size of the absorbed dose of radiation (Bik et al., 2003; Zagórski et al., 2011; Zagórski and Kornacka, 2013). Unique for radiation chemistry are primary phenomena. Absorption of ionizing radiation causes the detachment of electrons which travel to places of energetically preferential sites, e.g. positive holes

n Corresponding author. Tel.: þ 48 22 5041288, mobile: þ48 784222583; fax: þ48 22 5041313. E-mail address: [email protected] (W. Głuszewski).

http://dx.doi.org/10.1016/j.radphyschem.2014.06.024 0969-806X/& 2014 Elsevier Ltd. All rights reserved.

from previous ionizations or places where two chains are close to one another. The positive holes are also wandering along the chains, also to energetically preferable sites, other than those for electrons. Formation of excited states (as a result of electron capture by the hole) also takes place. All these phenomena, precedes the creation of a macroradical. Translation of reactive species takes important role in protective phenomena, if aromatic compounds are present (Głuszewski and Zagórski, 2008). In the case of elastomers, aromatics are both standard stabilizers (antioxidizers and photostabilizers) as well peroxidizers added for chemical crosslinking. Thermal treatment and radiation processing lower the amount of aromatics by degradation. Combined processes of radiation and chemical crosslinking can substantially influence the yield of the number of crosslinks and processes of oxidegradation of elastomere (Głuszewski et al., 2014). Under investigation was ethylene–octen thermoplastic elastomer of Engage type, crosslinked by peroxide and subsequently irradiated in the range of doses 20–300 kGy (Perraud et al., 2003; Mishra et al., 2008; Poongavalappil et al., 2013).

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2. Materials and methods Following sheets were prepared from elastomere Engage 8200 (Table 1), used for determination of the radiation yield of hydrogen and absorption of oxygen. Influence of thermal processes of formation and of the vulcanization and addition of peroxide on phenomena of oxidation in process of ageing were investigated Figs. 1–4. Properties of Engege 8200: Melt Index, 190 1C/2.16 kg, dg/min – (5.0), Density, g/cm3 – (0.870), Mooney Viscosity, ML 1 þ 4 @ 121 1C – (8), Ultimate Tensile Strength, MPa – (5.7), Ultimate Tensile Elongation – (1140), 100% Modulus, MPa – (2.3), Hardness, Shore A (1 s) – (66), Shore D (1 s) – (17), Tear Strength, Type C, kN/m – (37.1), Vicat Softening Point, 1C – (37), DSC Melting Point, 10 1C/min rate, 1C, Dow Method – (59), Glass Transition Temperature, 1C, Dow Method – (53), Tc Peak, 1C, Dow Method – (4). Fig. 3. The yield of hydrogen and the yield of absorbed oxygen vs absorbed dose. Table 1 Elastomeric materials used in the study. Engage 8200 (ethylene–octane copolymer (the DOW Chemical Company))

Marking

Granules Sheet without addition of peroxide Sheet with the addition of small amount of peroxide Sheet with higher (double in comparison to B) content of peroxide Vulcanized sheet without addition of peroxide Vulcanized sheet with peroxide Vulcanized sheet with double amount of peroxide

G A B C AV BC CV

Peroxides used: dicumul peroxide (Perkadox BC409, Akzo Nobel Polymer Chemicals) and Taic 50 (Kettlitz Chemie GmbH & C). Two types of samples were tested: 1% Perkadox BC409 and 0.9% Taic 50 and 2% Perkadox BC409 and 1.8% Taic 50. Samples were formed in a standard way by rolling and then forming (temperature, pressure) Tables 2–5. 2.1. Gas chromatography (GC) Gas chromatograph Shimadzu (thermal conductivity detector, molecular sieves 5A) was used for the determination of radiation yield of hydrogen evolution (GH2) and absorption of oxygen (GO2) from the dose of radiation in the range of 1–35 kGy. Samples were irradiated in air, in closed vessels with gas phase subjected to gas chromatographic analysis, at room temperature. The gas chromatograph was attached by interface to the PC computer where the data were acquired by program CHROMAX. The carrier gas was argon (99.99%), calibration gases were hydrogen 99.99% and oxygen 99.99%. Operations were done with syringes of volume 10, 25 and 500 µl. The chromatographic system was working at 220 1C, the column was kept at 40 1C and the detector at 100 1C. The rate of flow of carrier gas was 10 ml/min. Radiation efficiency was calculated in mmol/J. 2.2. Irradiation

Fig. 1. Volume of hydrogen formed in the result of irradiation of granulate and sheets of elastomers.

The samples were irradiated in air, at room temperature, at the source of gamma radiation, GC 5000, the Indian production of the dose rate was 7.0 kGy/h. Gamma radiation dose rate was measured with a Fricke dosimeter and alanine dosimeter. Dose distribution was measured with PVC foil. Samples were irradiated in air, in closed vessels with gas phase subjected to gas chromatographic analysis, at room temperature. 2.3. Diffuse reflection spectroscopy (DRS)

Fig. 2. The yield of hydrogen and the yield of absorbed oxygen vs absorbed dose.

One of advantages of diffuse reflection spectroscopy (DRS) is the possibility to investigate polymers in any shape. The principle of measurement consists in directing the beam of analyzing light on the surface of the sample. Part of light is reflected back unchanged, but another is bent into the sample and then inside reflections leave the sample with spectral information about compounds formed as the result of irradiation and/or compounds present before and destroyed. In our investigations the spectrophotometer JASCO V-670 equipped with reflection device was used. Several bands of absorbtion were identified. The band around 210 nm is acquired to peroxide groups. Bands at

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Fig. 4. Example of DRS spectrum of Engage 8200 elastomers.

Table 2 DRS spectra were compared to the same dose of radiation. Peak heights were compared to a wavelength of about 230 nm. Subsequent peaks, starting with smallest one

0 kGy

20 kGy

75 kGy

100 kGy

300 kGy

6 5 4 3 2 1

AV C B A CV BV

AV C B A BV CV

AV B A C BV CV

AV B A C CV BV

AV B CV A C BV

Table 3 The sheet vulcanized without peroxides (A). The radiation crosslinked sheet with doses of 60 and 120 kGy (B 60, B 120).

Table 4 The sheet with peroxides. Only vulcanized (W1). Vulcanized and radiation modification doses of 60 kGy (WR1). Only radiation crosslinked doses of 60 kGy. 8200 - 1 Property measured

W1

WR1 60 kGy

R1 60 kGy

Tensile Strength (MPa) Modulus 100% (MPa) Modulus 200% (MPa) Modulus 300% (MPa) Hardness Shore A (0ShA) Compression set % Permanent elongation % Compression set %

8.1 2 2.7 3.1 69 690 150 38.5

6.1 2.3 3.0 3.4 68 615 140 25

8.4 2.3 2.8 3.2 72 850 250 –

Table 5 The sheet with double in comparison to 8200 - 1 content of benzoyl peroxide. Only vulcanized (W2). Vulcanized and radiation modification doses of 120 kGy (WR2). Only radiation crosslinked doses of 60 kGy (R2). 8200 - 2

8200 Property measured

A

B 60 kGy

B 120 kGy

Property measured

W2

WR2 120 kGy

R2 60 kGy

Tensile strength (MPa) Modulus 100% (MPa) Modulus 200% (MPa) Modulus 300% (MPa) Hardness Shore A (0ShA) Elongation at break % Permanent elongation % Compression set %

6.9 2.3 2.8 3.1 71 1096 350 25.8

11.4 2 2.5 2.9 69 1046 – 25

10.1 2.1 2.7 3.0 68 776 200 25

Tensile strength (MPa) Modulus 100% (MPa) Modulus 200% (MPa) Modulus 300% (MPa) Hardness Shore A (0ShA) Elongation at break % Permanent elongation % Compression set %

5.1 2.2 2.8 3.4 70 477 100 37.5

4.1 2.5 3.2 3.9 68 327 40 24

9.9 2.3 2.8 3.1 72 1066 370 –

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295–320 nm are ascribed to CO groups at the end of chains (products of degradation) and peaks in the range of 245 to carboxyl groups in the middle of polymer chains. 3. Results and discussion Volume of hydrogen yield formed by radiolysis increases roughly proportional to the applied doses. For starting doses, up to 15 kGy, the increase is relatively higher. Only then, degradation of aromatic stabilizers causes an increase of macroradicals for higher doses, that means that hydrogen yield changes relatively less. Comparison for non-vulcanized sheets A, B and C shows the protective action of aromatic peroxide. Highest values of hydrogen yield were observed for the sheet without addition of crosslinking agent, the lowest one for double amount of peroxide. Vulcanization caused degradation of stabilizers, that is visible in the case of an increase of hydrogen yield for the sheet AV and lower one for sheets BV and CV. Even more visible is the protection effect in the dependence of oxygen yields from the dose. Relative relations between curves are similar as for hydrogen yields that can be explained by proportional dependence between GH2 for the sheet AV and lower one for BV and CV. Even more visible is the protection effect of aromatic peroxide in the case of GH2 depending on dose. Mutual relations between curves are analogical as in the case of GH2 that can be explained by proportional dependence between GH2 and the number of macroradicals, responsible for both crosslinking and oxidation.

irradiation, relatively best properties were reached for the sheet crosslinked by radiation. In the case of the sheet with double amount of peroxide, relatively best properties were obtained by radiation crosslinking. Radiation modification after proceeding vulcanization definitely increases modulus and reduces permanent elongation. Such process can be applied to strength second phase of preparation of elastomeric composites.

4. Conclusions Particular attention was devoted on the influence of protection effects exhibited by aromatic compounds added to the elastomers (peroxides, thermo- and photostabilizers), and on phenomena of radiation crosslinking and postirradtion oxidations. Aromatic peroxides can be modified during introductory irradiation, that influences the later process of vulcanization. They also influence phenomena of radiation that induced radiation crosslinking. Comparing the radiation crosslinked product without peroxide and with peroxide, better properties were shown for the product without peroxide. Most probably it is caused by protection effect of aromatic compounds. The role of peroxide in the process of crosslinking is not fully understood. Perhaps the energy transfer also causes crosslinking via decomposition of peroxide. From the experimental point of view, advantages of the DRS method stress its role in the investigation of oxidation processes on the surface of polymers.

3.1. Post-radiation oxidation of elastomers After-irradiation phenomena oxidation of elastomers were investigated by diffuse reflection spectrophotometry (Zagórski, 2003). Earlier papers on DRS were described by spectra from the point of view of products of oxidation of polymer chain. Sheets listed in Table 1 were irradiated by doses 20, 75, 100 and 300 kGy and later aged at the temperature 70 1C during 196 h. DRS spectra were measured against virgin samples, unirradiated and nonaged. It is clear that the height of bands increases with the absorbed dose of radiation. However, in the case of six curves, lowest intensities of peaks attributes to products of oxidation related to products with double amount of peroxide and vulcanizates with the addition of peroxide. The lowest resistance to radiation shows sheets without addition of peroxides, and vulcanized. 3.2. Mechanical properties The measurements (Tensile Strength, Modulus Hardness, Shore A, Elongation at break, Permanent elongation and Compression set) were performed in a certified laboratory at the Institute for Engineering of Polymer Materials and Dyes (PCA accreditation certificate No. AB 147 for the inspection of raw materials, blends and thermoplastic elastomer). The work was performed in accordance with Polish ISO standards. When compared, vulcanized product with peroxide crosslinked to the sheet by radiation and vulcanized product modified by

Acknowledgments Work was done under the research Project N N209083838 “synergistic system of crosslinking elastomers”, funded by the Ministry of Science and Higher Education. References Bik, J., Głuszewski, W., Rzymski, W.M., Zagórski, Z.P., 2003. EB radiation crosslinking of elastomers. Radiat. Phys. Chem. 67, 421–423. Głuszewski, W., Zagórski, Z.P., 2008. Radiation effects in polypropylene/polystyrene blends as the model of aromatic protection effects. Nukleonika 53, 21–24. Głuszewski, W., Zagórski, Z.P., Rajkiewicz, M., 2014. Synergistic effects in the processes of crosslinking of elastomers. Radiat. Phys. Chem. 94, 36–39. Mishra, J.K., Young-Wook Chang, Y.W., Lee, B.L., Sung Hun, Ryu, 2008. Mechanical properties and heat shrinkability of electron beam crosslinked polyethylene octene copolymer. Radiat. Phys. Chem. 77, 675–679. Perraud, S., Vallat, M.F., Kuczyński, J., 2003. Radiation crosslinking of poly(ethyleneco-octene) with electron beam radiation. Macromol. Mater. Eng. 288, 117–123. Poongavalappil, S., Svoboda, P., Theravalappil, Svobodova, Danek, M., Zatloukal, M., 2013. Study on the influence of electron beam irradiation on the thermal, mechanical, and rheological properties of ethylene–octene copolymer with high comonomer content. J. Appl. Polym. Sci., 3026–3033. Zagórski, Z.P., 2003. Diffuse reflection spectrophotometry for recognition of products of radiolysis of polymers. Int. J. Polym. Mater. 52, 323–333. Zagórski, Z.P., Kornacka, E.M., 2013. Radiation Processing of Elastomers. Advances in Elastomers I. Springer – Verlag, pp. 375–452. Zagórski, Z.P., Rajkiewicz, M., Głuszewski, W., 2011. Radiacyjna modyfikacja elastomerów. Przem. Chem. 6, 1191–1194.