CHAPTER 7 BLENDS OF NR WITH SPECIALITY RUBBERS MECHANICAL PROPERTIES AND AGING BEHAVIOR
7.1. Introduction There is ever increasing technical interest in the use of dissimilar rubbers in order to improve specific vulcanuate prcperties. eg. Ozone resistance, oil resistance, etc. Unfortunately, this can often be at the expense of reduction in other properties like, modulus, tensile strength, etc. of the compound. In the case of blends of ply diene elastomers with elastomers having lc,w olefinic content, the poor strength properties are thought to be a consequent of cure rate incompatibility between the polyrnerr.' Blends of NR with speciality elastomers have been used in many commercial applications.24 For getting better weather resistance NRI EPDM blends5' and for better oillfbel resistance NRNR have been used. Blending of butadiene rubber (BR) with NR"." improved its processing characteristic:^ and abrasion resistance. In this chapter, 50150 blends of NR with four speciality elastomers HNBR, EPDM, CR and NBR were prepared and the blends were evaluated for mechanical properties and aging tesistance with respect to heat, oil, ozone and radiation. Mixes of NR, NBR, CR, EPDM and WNBR were prepared as per formulation given in Table 7.1. 50150 blends of NR with NBR, CR, EPDM and HNBR were prepared by cross blending these mixes at the required ratio (method B) and compared the properties of the vulcanizates with that of NR.
Table 7.1. Compound formulation of mixes. Ingredients I NR EPDM HNBR Elastomer 100 Zinc Oxide 5 Steric acid 1 Sulphur 1.5 CBS" 1.5 MBT~ Magnesium oxide NA 22" DCP~ a Ncyclohexyl2bemthyazyl sulphenamide b Mercaptobemthiazole c Nisopropyl N'phenyl pphenylcne diamine d Dicumylperoxide 7.2. Cure characteristics Mixed cure systems were used for the blends. For EPDM and HNBR peroxide cure was selected, as these two wer.: saturated elastomers. For CR metal oxide cure and for NR and NBR sulfur cure were used. Figure 7.1 shows the rheographs of NR and blends of NR with NBR, CR. EPDN and HNBR at 160 "C. It was seen that blend of NR with HNBR gave highest torque and that with CR showed lowest torque. Cure characteristics were as given in Table 7.2. Scorch time of the blends were show to be lower than pure NR. Optimum curt: time was higher for the blends of NR with EPDM and HNBR, probably due to the pemxide cure of the EPDM and HNBR.
Figure 7.1. Rheographs of INR and 50150 blends of NR with 2NBR, 3CR, 4EPDM and 5 HNBR Table 7.2. Cure characteristics of NR and the blends at 160 BC MH, dnm 11.34 14.31.'. 13 3.12 ML, dnm 0.96 0.97 0.79 1.47 t90, min. 6.24 5.23 8,54 13.21 ts2, min. 5.24 3.52 4.43 1.89.6.99 1.22 14.50 1.94
7.3.Meehanical properties Mechanical properties of NR rnd the blends are given in Table 7.3. All the blends exhibited lower tensile strength than NR indicating lower level of crosslinking due to the thermodynamic incompatibility of thwe elastomers with NR as a result of the difference in their solubility parameter1 saturation level. Blends of NR with NBR and HNBR gave comparably higher tensile strength than other two blends. NRIEPDM blend showed very low tensile properties, probably due to the incompatibility between these two as a result of the wide difference in their saturirtion level l3 and also due to the lower gum tensile strength of EPDM. Better values of hardness and compression set were shown by the blends with EPDM and HNBR. NRIEPDM blend showed vety good high temperature compression set value while that of NR /CR was very poor. However, NRINBR blend showed better high temperature coml)ression set values than NR. Table 73. Mechanical properties of NR and the blends Properties Tensile strength, MPa Elongation at break, % Modulus loo%, MPa Modulus 300%, MPa Tear strength, kn1m Resilience, % Hardness, Shore A Compression set, % At 125 'C/ 22 h At 15O0C/22h NR 21.4 620 1.44 3.24 30.6 76 42 56.6 87.4
7.4. Aging characteristics 7.4. A. Thermal aging High performance and longterm heat and weather aging resistance are key requirements for many rubber applications. Figure 7.2 shows the retention of properties of NR and the blends after air aging at 150 C for 24 hours. NR and its blends with NBR'~ and CR'~ showed very poor thermal aging resistance as expected due to the unsaturation on their polymer backbone. Blends with HNBR showed better retention, whereas blends with EPDM exhibited very good resistance towards oxidation as a result of the stable saturated polymer back bone structure of these two elastomer^.'^'^ 90 80 U). w 73 5 60 n 0 2 a LC * a. NR NWNBR NWCR NWWM NWHNBR Figure.7.2. Percentage retention of properties after aging at 150 "C for 24 h.
7.4.B. Oil aging Figure 7.3 shows the retention of properties of NR and its blends after immersion in ASTM No.2 oil at room temperature for 5 days. A11 the blends showed improved aging resistance than NR. As was expected, blends with NBR and HNBR exhibited good resistance, whereas blends with CR and EPDM showed comparatively poor resistance. Figure 7.4 shows the photographs of the swelled samples. Table 7.4 gives the percentage swell of NR and the blends in ASTM oil Nos. 1, 2and 3 at three different temperatures room temperature, 708C and 1008C for 5 days. NR exhibited least resistance and even dissolved in the oils at higher temperature. Blend of NR with EPDM showed Least resistance among the blends, as EPDM contains no polar group and hence least resistant towards oils.20 Blends with NBR and HNBR exhibited very good oil resistance. NRLHNBR blend gave better resistance at higher temperature than FWNBR blend due to the excellent aging resistance of HNBR.~~ Selective hydrogenation of the C=C bonds, gave HNBR, excellent heat aging, while the nitrile group provided with very good oil/fuel re~istance.~~ NRICR exhibited better resistance than NR and NWEPDM vulcanizates, as CR contains polar group. 140 120.g,oo s a 0 & c. 0. 0 d r e 4 a4* 20 0 NR NWNBR NWCR NRlEPDM NWHNBR, Figure.7.3. Percentage retention of properties after aging in ASTM oil No:2 for 5 days at room temperature.
Figure.7.4. Photographs of oil swelled samples of NR and its blends in ASTM No: 2 oil for 5 days at room temperature.
~ Table 7.4. Percentage swelling of NH and its blends in ASTM Oils ( 5 days at three different temperatures) Temperature At room temperature At 70 C At 100 C ASTM NO. 1 2 3 1 2 3 1 2 3 NR 47.5 71.8 180.7 126.9 207.6 285.8 Dissolved >, >> NR/NBR 16.1 22.1 63.9 42.8 77.7 115.2 83.4 115.7 198.0 NRICR 24.9 39.6 147.4 95.5 141.7 272.2 94.O 138.9 279.0 DM 30.5 44.5 151.9 124.0 184.9 283.2 NRRINBR 15.6 22.3 54.7 33.6 682 101.9 182.9 204.3 409.0 79.5 99.8 143.0 7.4.C Ozone aging One of the main types of degradation of elastomer products in service under natural conditions is caused by atnlospheric ozone generated in nature by electrical discharge and also by solar radiation in the strat~s~here.~ Only a few pphm of ozone in air can cause cracking on surfaces which demolish the appearance and may destroy use ulness of the elastomer products. Ozonation and antioxidant efficiency of elastomers and blends had been extensively st~died.~~'~ Figure 7.5 shows the optical photographs of the ozone exposed slrrfaces of NR arld its blends in an ozone chamber at 50 pphm ozone concentration at 38.5 "C. Withiin th": first two hours itself cracks were formed on the surfaces ofnr and NWNBR blend and the deepening of the cracks was vety fast. Cracks were developed only after 16 h on the NRJCR blend and propagated slowly, whereas blends of NR with EPDM and HNBF. showed no cracks up to 50h of exposure.
Figure.7.5 Photographs of vulcanizates of 1. NR and its blends with 2. NBR, 3. CR, 4. EPDM and 5. HNBR after 40 h of exposure to ozone atmosphere. 7.4.D. Exposure to Sradiation Table 7.5 shows the retention of tensile properties of the blends after Sirradiation. All the vulcanizates showed decreased properties. It was seen that the properti6 were drastically reduced in NR and NWCR blends. NWEPDM exhibited higher retention of properties.28 NR/NBR and NRIHNBR showed comparative retention of property. Higher degradation occurred on prolonged irradiation. However, NR/HNBR showed
impmved modulus and elongation, indicating additional crosslinking along with degradation. Table 75. Tensile properties of vukanizates of NR and blends after y irradiation Properties NR I NRPJBR NRICR NRJEPDM NR/HNBR 15 M rad Tensile skngtb, MPa 7.6 12.7 6.7 4.0 12.5 Elongation at break, % 5 51 0 560 535 350 350 Modulus, loo%, MPa 1.07 1.64 0.97 1.37 1.53 Modulus, 300%, MPa 228 3.81 1.75 3.03 2.99 25 M rad Tensile strength, MPa 2.8 7.6 3.6 3.1 7.9 Elongation at break, % 4 15 500 390 385 410 Modulus, 1W%, MPa 0.64 0.99 0.72.917 1.31 Modulus, 300%, MPa 1.47 2.47 2.38 2.12 3.46 7.5. Conclusion Aging characteristics of 1\IR vulcanizates can be improved by blending with speciality elastomers like, CR, NBR, EPDM and HNBR. Among these NR/HNBR blend exhibited a combined effect of aging resistance towards heat, oil, ozone and radiation to a desirable level. NRIEPDM blend eventhough had good resistance towards ozone and
heat aging, showed very poor resistance towards oil aging and had very low tensile properties. NR/NBR blend showed better oil and radiation resistance, but exhibited very poor resistance towards heat and ozone. Blend of NRICR showed only slight improvement towards heat, oil and ozone. References 1. M.J.R. Loadman, A.J.Tinker, Rubber Chem.Technol., 62,232 (1989). 2. M.Kozolowski, J.Appl.Polym.Sci., 55, 1375 (1995). 3. J.Bhatt, B.K.Roy, A.K.Chandq S.K.Mustafi,P.K.Mohammed, Rubber India, 55(9),7 (2003). 4. C.S.L. Baker, I.R.Wallace, J.Nat. Rubb. Res., 1(4), 270 (1986). 5. N.M. Mathew, J.Polym.Sci.;k'olym.Lert., 22,135 (1984). 6. S.D.Tobimg, Rubber India, 4( 5),13 (1989). 7. N.Suma, R.Joseph, K.E.Geotge, J.Appl.Polym.Sci., 49(3), 147 (1993) 8. H. Ismail, S.Tan, B.T.Poli, J.Elasto.Plasti., 33(4), 251 (2001). 9. D.S.Campbell, A.V.Chapma1, I.R.Goodfield, W.S.Fulton, J.Nat..Rubb.Res., 7, 168 (1992). 10. K.G.Karnika de Silva, Micheal Lewan, J.Rubb.Res. Insti. Sri Lanka, 81,38 (1998). 11. D.Freitan de Castm, A.F.Marlins, J.C.M.Suarez, RC.Reis, Lly Visonte, Riode Janeiro, Kautsch GummiKurzstat., 56(1), 49 (2003). 12. B.Kuriakose, C.K.Premaletba, N.M.Mathew, Indian J.Nat.Rubb.Res., 3(1),1 (1990). 13. P.S.Brown, A.J.Tinker, J. Nrt.Rubb.Res., 5(3), 157 (1990). 14. P.Budxugeac, Polym. Degrad Stabil., 47(10, 129 (1995). 15. P. Budrugeac, S.Ciutacu, Polym. Degrad. Stabil., 33 (3). 377 (1991). 16. G.Bmwes, "Elastomers and their applications", Pmce.165th meeting of ACS Rubb.Div., 1719 th May 2004, pp25, Akron, Ohio. 17. Arnis U. Paeglis, Rubber Cl~em.Technol., 77(2),242 (2004).