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Studies on polyisobutylene bound paraphenylene diamine antioxidant in natural rubber
Polymer Degradation and Stability 63 (1999) 225±230
Studies on polyisobutylene bound paraphenylene diamine antioxidant in natural rubber P.B. Sulekha, R. Joseph*, K.E. George Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin-682 022, India Received 27 March 1998; received in revised form 19 May 1998; accepted 7 June 1998
Abstract Paraphenylene diamine was chemically attached to low molecular weight chlorinated polyisobutylene.The polymer bound paraphenylene diamine was characterised by VPO, 1H NMR, IR and TGA. The eciency and permanence of the polymer bound paraphenylene diamine was compared with conventional amine type antioxidant in natural rubber vulcanizates. The vulcanizates showed improved ageing resistance and ozone resistance in comparison to vulcanizates containing conventional antioxidant. The presence of liquid, polymer-bound paraphenylene diamine reduces the amount of plasticiser required for compounding. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The technological use of both natural and synthetic rubber is intimately related to the use of antioxidants or protective agents in inhibiting the oxidation of rubber by atmospheric oxygen. The serious disadvantages of conventional antioxidants are their volatility and extractability in water or other solvents. The removal of antioxidants from the polymer during service has been shown to have an adverse eect on ageing and useful life of the products [1]. One method to overcome this problem is to develop polymer bound antioxidants. There are two basic approaches to obtain polymeric antidegradants. One is to copolymerise various monomers having an antioxidative active moiety with elastomeric monomers. The other is by the direct combination of the conventional antioxidant with modi®ed elastomers [2,3]. Polymer bound antioxidants are highly resistant to volatilisation and extraction [4]. Polymerisable monomeric antioxidants were described by Tamura and coworkers [5,6]. The reaction of chloroprene (CR) type-w with amines producing polymer bound antioxidants were reported by Al-Mehadawe et al. [7]. Scott et al. have demonstrated that simple hindered phenols which contain a methyl group in the ortho or para position, can react with natural rubber in * Corresponding author. Tel.: +91-0484-55723; fax: +91-0484532495; e-mail: [email protected]
the presence of oxidising free radicals to yield polymer bound antioxidants [8±10]. Antioxidants like styrenated phenol, diphenylamine etc bound to hydroxy terminated liquid natural rubber by a modi®ed Friedel± Crafts reaction were also found to be eective in improving ageing resistance [11]. Natural rubber bound diphenylamine antioxidants were reported by Avirah and Joseph [12]. Polymer bound antioxidants have many advantages but one main disadvantage is that the mobility of the network-bound antioxidants is restricted. Hence it does not bloom to the surface and ozone attack cannot be eectively controlled [13]. Most of the polymer bound antioxidants prepared have an unsaturated backbone. So during the process of vulcanisation their backbone gets attached to the main chain through sulphur crosslinking and it loses its mobility still further [14]. This paper describes the chemical binding of paraphenylene diamine to chlorinated low molecular weight polyisobutylene (PIB). Thus a polymer bound antioxidant with saturated backbone was prepared. 2. Experimental 2.1. Materials Polyisobutylene (PIB) with molecular weight 934 was supplied by Cochin Re®neries, Balmer Lawrie Ltd
0141-3910/99/$Ðsee front matter # 1999 Elsevier Science Ltd. All rights reserved PII: S0141 -3 910(98)00096 -2
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P.B. Sulekha et al./Polymer Degradation and Stability 63 (1999) 225±230
Ambalamugal, Kerala. Natural rubber (NR) (ISNR-5, Mooney viscosity ML(1+4) 100 C-82) was supplied by the Rubber Research Institute of India, Kottayam. Compounding ingredients zinc oxide, stearic acid, aromatic oil and carbon black (HAF N 330), were commercial grade, mercaptobenzothiazole (MBT) and tetramethyl thiuramdisulphide (TMTD) were rubber gradeÐsupplied by Bayer India Ltd. Methanol, carbon tetrachloride, ether, toluene, dioxan, triethylamine were of reagent grade and used as such. Paraphenylene diamine (PD) (Analar grade) from E. Merck India was used. Commercial antioxidant, Pil¯ex-13 (substituted paraphenylene diamine) was supplied by Poly Ole®ns Ltd, India. 2.2. Preparation of polyisobutylene bound PD Polyisobutylene (PIB) was dissolved in CCl4 (50% solution) and pure dry chlorine gas was passed through it for 5 h. The resulting solution was poured into water at 80 C and excess chlorine and CCl4 were removed. The chlorinated sample was dissolved in toluene and reprecipitated using methanol. The sample was dried in a vacuum oven. The chlorinated sample was dissolved in dioxan and mixed with PD dissolved in dioxan in the ratio 2:1. The reaction mixture was made alkaline by adding triethylamine, to neutralise the HCl evolved during the condensation reaction. The reaction mixture was taken in a two necked ¯ask ®tted with a water condenser and a thermometer. The mixture was heated at 100 C for 48 h. Dioxan and triethylamine were distilled o and the mixture was washed with methanol a number of times in order to remove the unreacted PD. The resulting mixture was reprecipitated using a toluene±methanol (1:1 v/v) mixture and the product dried in a vacuum oven. 2.3. Analysis of bound antioxidant Analysis of bound antioxidant was carried out by using infrared spectroscopy (IR), proton magnetic resonance spectroscopy (1H NMR), vapour phase osmometry (VPO) and thermogravimetric analysis (TGA). IR spectra of the samples were taken on a PerkinElmer Model 377 IR Spectrometer. The 1H NMR spectra of the samples were recorded from a solution in CCl4 using a JEOL-JNM spectrometer. TGA was carried out using a Dupont TG-DSC standard model in nitrogen atmosphere at a heating rate of 10 C/min. Molecular weight was determined by using Knauer Vapour Phase Osmometer. The optimum concentration of the rubber bound antioxidant for attaining maximum retention in properties was determined by varying the amount of the antioxidant in the mix from 1 to 10 ph.
The chemically bound paraphenylene diamine was added in NR as per formulations given in Table 1. The amount of the plasticiser can be reduced by the use of liquid polymer bound antioxidant as shown in Table 1. The optimum cure times (time to reach 90% of the maximum torque) of compounds were determined on a GoÈttfert elastograph, model 67.85, as per ASTM D1646 (1981). Rubber compounds were moulded in an electrically heated laboratory hydraulic press at 150 C up to their optimum cure time. Dumbell shaped tensile test pieces were punched out of these compression moulded sheets along the mill grain direction. The tensile properties of the vulcanizates were evaluated on a Zwick universal testing machine using a cross head speed of 500 mm/ min according to ASTM D-412-80. Tear resistance of the vulcanizates was evaluated as per ASTM D-2240. Retention in tensile and tear properties were evaluated after ageing the samples at 100 C for 12, 24, 36 and 48 h. The compression set of the samples was determined as per ASTM D-39 methods. A Goodrich ¯exometer conforming to ASTM D 62367 Method A was used for measuring heat build up. The test was carried out with a cylindrical sample of 2.5 cm in height and 1.9 cm in diameter. The oven temperature was kept constant at 50 C. The stroke was adjusted to 4.45 mm and load of 10.9 kg. Ozone ageing studies under static conditions were conducted according to ASTM D 518 Method D in a Mast Model 700-1 ozone test chamber at 41 C. Ozone concentration in the chamber was adjusted to 50 parts per hundred million (pphm). 3. Results and discussion Figs. 1±3 show the IR spectra of PIB, PIB±Cl and PIB±PD. The IR Spectra of PIB shows peaks at 2950 cmÿ1 corresponding to aliphatic ±CH stretching, at 1480 cmÿ1 due to ±CH bending, 1650 cmÿ1 corresponds to C C. The IR spectrum of PIB±Cl (chlorinated PIB) shows a new peak at 780 cmÿ1 due to the presence of C± Table 1 Formulations for antioxidant testing Sample Natural rubber Zinc oxide Stearic acid Carbon black (HAF, N330) Mercaptobenzothiazole Tetramethylthiuramdisulphide Aromatic oil Sulphur Pil¯ex 13 PIB±PD
X WA A 100 5.0 2.0 40.0 0.8 0.2 5.0 2.5 1.0 ±
100 5.0 2.0 40.0 0.8 0.2 5.0 2.5 ± ±
100 5.0 2.0 40.0 0.8 0.2 3.0 2.5 ± 2.0
B
C
D
E
100 5.0 2.0 40.0 0.8 0.2 1.0 2.5 ± 4.0
100 5.0 2.0 40.0 0.8 0.2 ± 2.5 ± 6.0
100 5.0 2.0 40.0 0.8 0.2 ± 2.5 ± 8.0
100 5.0 2.0 40.0 0.8 0.2 ± 2.5 ± 10.0
P.B. Sulekha et al./Polymer Degradation and Stability 63 (1999) 225±230
Cl stretching. The IR spectra of PIB±PD shows additional peaks at 3390 cmÿ1 due to NH stretching, 1620 cmÿ1 due to NH bending and 1500 cmÿ1 due to the aromatic ring present in PD. This con®rms the chemical binding of PD on to PIB [15]. Figs. 4 and 5 show the 1H NMR spectra of PIB and PIB±PD. There are peaks at =1.22 (±CH2) and =1.6
227
ppm (±CH3). Fig. 5 shows additional peaks at =7.2 ppm and =3.5 ppm corresponding to aromatic ring and Ar±NH±R present in PIB±PD. This again con®rms the chemical binding of PD to PIB [14]. The molecular weight of the samples were determined by VPO. The molecular weights of PIB, PIB±Cl, PIB± PD and PD were 930, 1000, 2300 and 110 respectively. This shows that polyisobutylene is bound to paraphenylene diamine at both ends. The chlorine content of the PIB±Cl is found to be 7.6%. This shows that addition reaction has taken place at the double bond. Fig. 6 shows the thermograms of PD, PIB, and PIB± PD. The low molecular weight PD volatilises easily while rubber bound PD is less volatile. This again con®rms the chemical bonding of PD onto PIB. From the above data the chlorination and binding reaction between PIB and PD may be as shown below
Fig. 1. IR spectrum of PIB.
Fig. 2. IR spectrum of PIB±CI.
Fig. 3. IR spectrum of PIB±PD.
Fig. 7 shows the variation in tensile strength after ageing at 100 C for 48 h with exposure time after the addition PIB±PD to NR vulcanizate. The retention in properties is found to increase with the amount of the bound antioxidant, reach a maximum and then decrease. The decrease in tensile strength may be due to incompatibility of the bound antioxidant in NR at higher concentration.
Fig. 4. 1H NMR spectrum of PIB.
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P.B. Sulekha et al./Polymer Degradation and Stability 63 (1999) 225±230
Fig. 5. 1H NMR spectrum of PIB±PD.
Fig. 7. Variation of tensile strength with amount of antioxidant.
Fig. 6. TGA of compounds: A, PIB±PD; B, PIB±Cl; C, PD.
Fig. 8 shows the tensile strength of the vulcanizates of compounds shown in Table 1 before and after ageing. All the vulcanizates show fairly good resistance to ageing at 100 C for 24 h but only compounds containing PIB±PD show good resistance when the ageing time is increased to 48 h, which shows the superiority of the bound antioxidant over the conventional antioxidant. Fig. 9 shows the change in elongation at break of the vulcanizates before and after ageing. The compound containing PIB±PD shows better retention in elongation at break after ageing. This again shows that the bound antioxidant can improve the ageing resistance of the NR compound.
Fig. 8. Variation of tensile strength of the vulcanizates before and after ageing. WAÐwithout antioxidant, AÐ2 g, BÐ4 g, CÐ6 g and DÐ8 g of PIB±PD respectively; XÐ1 g of pil¯ex 13.
Plates 1 and 2 show ozone cracked samples of conventional and polymer bound antioxidant respectively. The crack initiation of the samples took place after 2 h 35 min. The total failure of the vulcanizate containing
P.B. Sulekha et al./Polymer Degradation and Stability 63 (1999) 225±230
229
Plate 2. Ozone cracked surface of the vulcanizate containing PIB±PD after 14 h 14 min.
Table 2 Fig. 9. Variation of elongation at break of the vulcanizates before and after ageing. DÐ8 g of PIB±PD, XÐ1 g of pil¯ex 13, WAÐwithout antioxidant.
Sample Heat build up
Pil¯ex 13 (X)
PIB±PD (D)
WA
19.5
19
25
4. Conclusions
Plate 1. Ozone cracked surface of the vulcanizate containing pil¯ex 13 after 7 h 10 min.
pil¯ex 13 took place after 7 h 10 min, while that of PIB± PD took place after 14 h 40 min. This indicates that the ozone resistance of the vulcanizates containing PIB±PD is superior to that of vulcanizates containing pil¯ex 13. PIB±PD has a saturated backbone and its mobility is not lost during vulcanization and so it has a superior ozone resistance. The heat build up values of vulcanizates containing conventional and bound antioxidant are almost the same, as shown in Table 2. This shows that vulcanizate containing PIB±PD is comparable with conventional antioxidant.
1. PD can be chemically bound to chlorinated polyisobutylene by condensation reaction. 2. The rubber bound antioxidant has much superior resistance to evaporation compared to conventional antioxidant. 3. The rubber bound antioxidant can improve the ageing resistance of NR vulcanizate. 4. The rubber bound antioxidant can reduce the amount of plasticizer required for compounding. 5. The ozone resistance of the vulcanizate containing PIB±PD is superior to that of the vulcanizate containing pil¯ex 13 and the heat build up remains unchanged.
References [1] Thomas KD. In: Scott G, editor. Developments in polymer stabilisationÐ1. London; Applied Science Publishers, 1979:137. [2] Scott G. In: Scott G, editor. Developments in polymer stabilisationÐ4. London: Applied Science Publishers, 1981:181. [3] PospõÂsÏ il J. In: Scott G, editor. Developments in polymer stabilisationÐ7. London: Applied Science Publishers, 1984:53. [4] Grassie N, Scott G. Polymer degradation and stabilisation. Cambridge: Cambridge University Press, 1985:156. [5] Nippon Zeon Co., US Patent 4298522, 1981. [6] Nippon Zeon Co., British Patent 2053911, 1980.
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P.B. Sulekha et al./Polymer Degradation and Stability 63 (1999) 225±230
[7] Al-Mehadawe MS, Stuckey JE. Rubber Chemistry and Technology 1989;62:13. [8] Amarapathy AMA, Scott G. Mechanisms of antioxidant action. Improved ageing performance of latex products containing bound antioxidants. Presented at International Polymer Latex Conference, London, 31 October±2 November 1978. [9] Kularatne K, Sirimevan W, Scott G. European Polymer Journal 1979;14:827. [10] Scott G. US Patent 4213892, 1980.
[11] Avirah S, Joseph R. Angew Macromol Chem 1991:193. [12] Avirah S, Joseph R. Polymer Degradation and Stability 1994;46:251±7. [13] Avirah S, Joseph R. Journal of Applied Polymer Science 1995;57:1511±1524. [14] Meyrick TJ. In: Blow CM, Hepburn C, editors. Rubber technology and manufacture. Butterworths, 1982:244. [15] Dyer JR. Applications of absorption spectroscopy of organic compounds. New Jersey: Prentice Hall, 1984:23.
Studies on polyisobutylene bound paraphenylene diamine antioxidant in natural rubber P.B. Sulekha, R. Joseph*, K.E. George Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin-682 022, India Received 27 March 1998; received in revised form 19 May 1998; accepted 7 June 1998
Abstract Paraphenylene diamine was chemically attached to low molecular weight chlorinated polyisobutylene.The polymer bound paraphenylene diamine was characterised by VPO, 1H NMR, IR and TGA. The eciency and permanence of the polymer bound paraphenylene diamine was compared with conventional amine type antioxidant in natural rubber vulcanizates. The vulcanizates showed improved ageing resistance and ozone resistance in comparison to vulcanizates containing conventional antioxidant. The presence of liquid, polymer-bound paraphenylene diamine reduces the amount of plasticiser required for compounding. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction The technological use of both natural and synthetic rubber is intimately related to the use of antioxidants or protective agents in inhibiting the oxidation of rubber by atmospheric oxygen. The serious disadvantages of conventional antioxidants are their volatility and extractability in water or other solvents. The removal of antioxidants from the polymer during service has been shown to have an adverse eect on ageing and useful life of the products [1]. One method to overcome this problem is to develop polymer bound antioxidants. There are two basic approaches to obtain polymeric antidegradants. One is to copolymerise various monomers having an antioxidative active moiety with elastomeric monomers. The other is by the direct combination of the conventional antioxidant with modi®ed elastomers [2,3]. Polymer bound antioxidants are highly resistant to volatilisation and extraction [4]. Polymerisable monomeric antioxidants were described by Tamura and coworkers [5,6]. The reaction of chloroprene (CR) type-w with amines producing polymer bound antioxidants were reported by Al-Mehadawe et al. [7]. Scott et al. have demonstrated that simple hindered phenols which contain a methyl group in the ortho or para position, can react with natural rubber in * Corresponding author. Tel.: +91-0484-55723; fax: +91-0484532495; e-mail: [email protected]
the presence of oxidising free radicals to yield polymer bound antioxidants [8±10]. Antioxidants like styrenated phenol, diphenylamine etc bound to hydroxy terminated liquid natural rubber by a modi®ed Friedel± Crafts reaction were also found to be eective in improving ageing resistance [11]. Natural rubber bound diphenylamine antioxidants were reported by Avirah and Joseph [12]. Polymer bound antioxidants have many advantages but one main disadvantage is that the mobility of the network-bound antioxidants is restricted. Hence it does not bloom to the surface and ozone attack cannot be eectively controlled [13]. Most of the polymer bound antioxidants prepared have an unsaturated backbone. So during the process of vulcanisation their backbone gets attached to the main chain through sulphur crosslinking and it loses its mobility still further [14]. This paper describes the chemical binding of paraphenylene diamine to chlorinated low molecular weight polyisobutylene (PIB). Thus a polymer bound antioxidant with saturated backbone was prepared. 2. Experimental 2.1. Materials Polyisobutylene (PIB) with molecular weight 934 was supplied by Cochin Re®neries, Balmer Lawrie Ltd
0141-3910/99/$Ðsee front matter # 1999 Elsevier Science Ltd. All rights reserved PII: S0141 -3 910(98)00096 -2
226
P.B. Sulekha et al./Polymer Degradation and Stability 63 (1999) 225±230
Ambalamugal, Kerala. Natural rubber (NR) (ISNR-5, Mooney viscosity ML(1+4) 100 C-82) was supplied by the Rubber Research Institute of India, Kottayam. Compounding ingredients zinc oxide, stearic acid, aromatic oil and carbon black (HAF N 330), were commercial grade, mercaptobenzothiazole (MBT) and tetramethyl thiuramdisulphide (TMTD) were rubber gradeÐsupplied by Bayer India Ltd. Methanol, carbon tetrachloride, ether, toluene, dioxan, triethylamine were of reagent grade and used as such. Paraphenylene diamine (PD) (Analar grade) from E. Merck India was used. Commercial antioxidant, Pil¯ex-13 (substituted paraphenylene diamine) was supplied by Poly Ole®ns Ltd, India. 2.2. Preparation of polyisobutylene bound PD Polyisobutylene (PIB) was dissolved in CCl4 (50% solution) and pure dry chlorine gas was passed through it for 5 h. The resulting solution was poured into water at 80 C and excess chlorine and CCl4 were removed. The chlorinated sample was dissolved in toluene and reprecipitated using methanol. The sample was dried in a vacuum oven. The chlorinated sample was dissolved in dioxan and mixed with PD dissolved in dioxan in the ratio 2:1. The reaction mixture was made alkaline by adding triethylamine, to neutralise the HCl evolved during the condensation reaction. The reaction mixture was taken in a two necked ¯ask ®tted with a water condenser and a thermometer. The mixture was heated at 100 C for 48 h. Dioxan and triethylamine were distilled o and the mixture was washed with methanol a number of times in order to remove the unreacted PD. The resulting mixture was reprecipitated using a toluene±methanol (1:1 v/v) mixture and the product dried in a vacuum oven. 2.3. Analysis of bound antioxidant Analysis of bound antioxidant was carried out by using infrared spectroscopy (IR), proton magnetic resonance spectroscopy (1H NMR), vapour phase osmometry (VPO) and thermogravimetric analysis (TGA). IR spectra of the samples were taken on a PerkinElmer Model 377 IR Spectrometer. The 1H NMR spectra of the samples were recorded from a solution in CCl4 using a JEOL-JNM spectrometer. TGA was carried out using a Dupont TG-DSC standard model in nitrogen atmosphere at a heating rate of 10 C/min. Molecular weight was determined by using Knauer Vapour Phase Osmometer. The optimum concentration of the rubber bound antioxidant for attaining maximum retention in properties was determined by varying the amount of the antioxidant in the mix from 1 to 10 ph.
The chemically bound paraphenylene diamine was added in NR as per formulations given in Table 1. The amount of the plasticiser can be reduced by the use of liquid polymer bound antioxidant as shown in Table 1. The optimum cure times (time to reach 90% of the maximum torque) of compounds were determined on a GoÈttfert elastograph, model 67.85, as per ASTM D1646 (1981). Rubber compounds were moulded in an electrically heated laboratory hydraulic press at 150 C up to their optimum cure time. Dumbell shaped tensile test pieces were punched out of these compression moulded sheets along the mill grain direction. The tensile properties of the vulcanizates were evaluated on a Zwick universal testing machine using a cross head speed of 500 mm/ min according to ASTM D-412-80. Tear resistance of the vulcanizates was evaluated as per ASTM D-2240. Retention in tensile and tear properties were evaluated after ageing the samples at 100 C for 12, 24, 36 and 48 h. The compression set of the samples was determined as per ASTM D-39 methods. A Goodrich ¯exometer conforming to ASTM D 62367 Method A was used for measuring heat build up. The test was carried out with a cylindrical sample of 2.5 cm in height and 1.9 cm in diameter. The oven temperature was kept constant at 50 C. The stroke was adjusted to 4.45 mm and load of 10.9 kg. Ozone ageing studies under static conditions were conducted according to ASTM D 518 Method D in a Mast Model 700-1 ozone test chamber at 41 C. Ozone concentration in the chamber was adjusted to 50 parts per hundred million (pphm). 3. Results and discussion Figs. 1±3 show the IR spectra of PIB, PIB±Cl and PIB±PD. The IR Spectra of PIB shows peaks at 2950 cmÿ1 corresponding to aliphatic ±CH stretching, at 1480 cmÿ1 due to ±CH bending, 1650 cmÿ1 corresponds to C C. The IR spectrum of PIB±Cl (chlorinated PIB) shows a new peak at 780 cmÿ1 due to the presence of C± Table 1 Formulations for antioxidant testing Sample Natural rubber Zinc oxide Stearic acid Carbon black (HAF, N330) Mercaptobenzothiazole Tetramethylthiuramdisulphide Aromatic oil Sulphur Pil¯ex 13 PIB±PD
X WA A 100 5.0 2.0 40.0 0.8 0.2 5.0 2.5 1.0 ±
100 5.0 2.0 40.0 0.8 0.2 5.0 2.5 ± ±
100 5.0 2.0 40.0 0.8 0.2 3.0 2.5 ± 2.0
B
C
D
E
100 5.0 2.0 40.0 0.8 0.2 1.0 2.5 ± 4.0
100 5.0 2.0 40.0 0.8 0.2 ± 2.5 ± 6.0
100 5.0 2.0 40.0 0.8 0.2 ± 2.5 ± 8.0
100 5.0 2.0 40.0 0.8 0.2 ± 2.5 ± 10.0
P.B. Sulekha et al./Polymer Degradation and Stability 63 (1999) 225±230
Cl stretching. The IR spectra of PIB±PD shows additional peaks at 3390 cmÿ1 due to NH stretching, 1620 cmÿ1 due to NH bending and 1500 cmÿ1 due to the aromatic ring present in PD. This con®rms the chemical binding of PD on to PIB [15]. Figs. 4 and 5 show the 1H NMR spectra of PIB and PIB±PD. There are peaks at =1.22 (±CH2) and =1.6
227
ppm (±CH3). Fig. 5 shows additional peaks at =7.2 ppm and =3.5 ppm corresponding to aromatic ring and Ar±NH±R present in PIB±PD. This again con®rms the chemical binding of PD to PIB [14]. The molecular weight of the samples were determined by VPO. The molecular weights of PIB, PIB±Cl, PIB± PD and PD were 930, 1000, 2300 and 110 respectively. This shows that polyisobutylene is bound to paraphenylene diamine at both ends. The chlorine content of the PIB±Cl is found to be 7.6%. This shows that addition reaction has taken place at the double bond. Fig. 6 shows the thermograms of PD, PIB, and PIB± PD. The low molecular weight PD volatilises easily while rubber bound PD is less volatile. This again con®rms the chemical bonding of PD onto PIB. From the above data the chlorination and binding reaction between PIB and PD may be as shown below
Fig. 1. IR spectrum of PIB.
Fig. 2. IR spectrum of PIB±CI.
Fig. 3. IR spectrum of PIB±PD.
Fig. 7 shows the variation in tensile strength after ageing at 100 C for 48 h with exposure time after the addition PIB±PD to NR vulcanizate. The retention in properties is found to increase with the amount of the bound antioxidant, reach a maximum and then decrease. The decrease in tensile strength may be due to incompatibility of the bound antioxidant in NR at higher concentration.
Fig. 4. 1H NMR spectrum of PIB.
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Fig. 5. 1H NMR spectrum of PIB±PD.
Fig. 7. Variation of tensile strength with amount of antioxidant.
Fig. 6. TGA of compounds: A, PIB±PD; B, PIB±Cl; C, PD.
Fig. 8 shows the tensile strength of the vulcanizates of compounds shown in Table 1 before and after ageing. All the vulcanizates show fairly good resistance to ageing at 100 C for 24 h but only compounds containing PIB±PD show good resistance when the ageing time is increased to 48 h, which shows the superiority of the bound antioxidant over the conventional antioxidant. Fig. 9 shows the change in elongation at break of the vulcanizates before and after ageing. The compound containing PIB±PD shows better retention in elongation at break after ageing. This again shows that the bound antioxidant can improve the ageing resistance of the NR compound.
Fig. 8. Variation of tensile strength of the vulcanizates before and after ageing. WAÐwithout antioxidant, AÐ2 g, BÐ4 g, CÐ6 g and DÐ8 g of PIB±PD respectively; XÐ1 g of pil¯ex 13.
Plates 1 and 2 show ozone cracked samples of conventional and polymer bound antioxidant respectively. The crack initiation of the samples took place after 2 h 35 min. The total failure of the vulcanizate containing
P.B. Sulekha et al./Polymer Degradation and Stability 63 (1999) 225±230
229
Plate 2. Ozone cracked surface of the vulcanizate containing PIB±PD after 14 h 14 min.
Table 2 Fig. 9. Variation of elongation at break of the vulcanizates before and after ageing. DÐ8 g of PIB±PD, XÐ1 g of pil¯ex 13, WAÐwithout antioxidant.
Sample Heat build up
Pil¯ex 13 (X)
PIB±PD (D)
WA
19.5
19
25
4. Conclusions
Plate 1. Ozone cracked surface of the vulcanizate containing pil¯ex 13 after 7 h 10 min.
pil¯ex 13 took place after 7 h 10 min, while that of PIB± PD took place after 14 h 40 min. This indicates that the ozone resistance of the vulcanizates containing PIB±PD is superior to that of vulcanizates containing pil¯ex 13. PIB±PD has a saturated backbone and its mobility is not lost during vulcanization and so it has a superior ozone resistance. The heat build up values of vulcanizates containing conventional and bound antioxidant are almost the same, as shown in Table 2. This shows that vulcanizate containing PIB±PD is comparable with conventional antioxidant.
1. PD can be chemically bound to chlorinated polyisobutylene by condensation reaction. 2. The rubber bound antioxidant has much superior resistance to evaporation compared to conventional antioxidant. 3. The rubber bound antioxidant can improve the ageing resistance of NR vulcanizate. 4. The rubber bound antioxidant can reduce the amount of plasticizer required for compounding. 5. The ozone resistance of the vulcanizate containing PIB±PD is superior to that of the vulcanizate containing pil¯ex 13 and the heat build up remains unchanged.
References [1] Thomas KD. In: Scott G, editor. Developments in polymer stabilisationÐ1. London; Applied Science Publishers, 1979:137. [2] Scott G. In: Scott G, editor. Developments in polymer stabilisationÐ4. London: Applied Science Publishers, 1981:181. [3] PospõÂsÏ il J. In: Scott G, editor. Developments in polymer stabilisationÐ7. London: Applied Science Publishers, 1984:53. [4] Grassie N, Scott G. Polymer degradation and stabilisation. Cambridge: Cambridge University Press, 1985:156. [5] Nippon Zeon Co., US Patent 4298522, 1981. [6] Nippon Zeon Co., British Patent 2053911, 1980.
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P.B. Sulekha et al./Polymer Degradation and Stability 63 (1999) 225±230
[7] Al-Mehadawe MS, Stuckey JE. Rubber Chemistry and Technology 1989;62:13. [8] Amarapathy AMA, Scott G. Mechanisms of antioxidant action. Improved ageing performance of latex products containing bound antioxidants. Presented at International Polymer Latex Conference, London, 31 October±2 November 1978. [9] Kularatne K, Sirimevan W, Scott G. European Polymer Journal 1979;14:827. [10] Scott G. US Patent 4213892, 1980.
[11] Avirah S, Joseph R. Angew Macromol Chem 1991:193. [12] Avirah S, Joseph R. Polymer Degradation and Stability 1994;46:251±7. [13] Avirah S, Joseph R. Journal of Applied Polymer Science 1995;57:1511±1524. [14] Meyrick TJ. In: Blow CM, Hepburn C, editors. Rubber technology and manufacture. Butterworths, 1982:244. [15] Dyer JR. Applications of absorption spectroscopy of organic compounds. New Jersey: Prentice Hall, 1984:23.