New uracil derivatives as antioxidants for natural rubber

New uracil derivatives as antioxidants for natural rubber E.M.A. Yakout Department of Pesticide Chemistry, National Research Centre, Cario, Egypt, and

S.H. El-Sabbagh Department of Polymers and Pigments, National Research Centre, Cairo, Egypt Abstract Purpose – Evaluation of uracil and/or benzothiazol derivatives as antioxidants in natural rubber mixes. Design/methodology/approach – Cyanoacetylurea 1, as a precursor, was prepared at a good yield from widely available, low-cost chemicals. Compound 1 was treated with triethylorthoformate and amine derivatives in one pot reaction affording the target uracil derivative 3. Replacement of the cyano group in 1 by benzothiazol led to obtaining the interesting N-hydroxy uracils containing benzothiazole moiety 5 at a good yield. Some of the compounds prepared was selected and were evaluated as antioxidants in natural rubber mixes. The rheometric characteristic of the compounded rubber and the physico-mechanical properties of the vulcanizates were determined. Findings – The cure rate index, tensile strength and modulus increased while the equilibrium swelling decreased, i.e. compound 5 behaved as a secondary accelerator. The rubber vulcanizates were subjected to thermal oxidative ageing at 908C for up to seven days. It has been found that uracil and/or benzothiazol derivatives can protect natural rubber vulcanizates against oxidative deterioration. Research limitations/implications – The compounds prepareds were difficult to dissolve, they needed solvents with high boiling points, e.g. DMF, DMSO to be dissolved and even then they are not completely dissolved. Practical implications – Uracil and or benzothiazol derivatives have many industrial applications. Originality/value – The new compounds were prepared from very cheap and widely available chemicals. The compounds synthesised showed good antioxidant behaviour in comparison with the commercial antioxidant (phenyl-b-naphthyl amine) industrially used. Keywords Natural rubber, Oxidation resistance Paper type Research paper

On the other hand, synthesis of organic compounds and evaluation of their efficiency in rubber mixes as accelerators/ antioxidants still create increasing interest. In spite of the fact that organic compounds which can be used as accelerators/ antioxidants and anti-fatigue agents are added in small amounts (1-2 phr), e.g. phenolics, amines, organophosphorus and organo-sulfur compounds, they play a very important role in determining the service characteristics of the resulting rubber vulcanizates. Consequently, the elaboration of heat resistance rubbers is considered to be a very important national task (Ismail et al., 2002, 1993; Youssef et al., 2003; Hann et al., 1994). The efficiency of the accelerators/antioxidants greatly depends on different factors such as rubber type, type of ageing and the nature of other rubber ingredients in the mixes and also on their chemical structure (Morton, 1973). It has been reported that some heterocylic compounds, such as pyridazine derivatives, are useful as antioxidants for rubber vulcanizates (Ismail et al., 2002, 1992; Yehia et al., 2002). Another important class of sulphur containing antioxidants is the group of metal complexes derived from thiols and organophosphorus compounds such as mercapto-benzothiazole and mercaptobenzimidazole (Ismail et al., 2002). The aim of the present work was to prepare uracil and uracil benzothiazole derivatives and evaluate their efficiencies as secondary accelerators and also as antioxidant agents in natural rubber (NR).

Introduction Many uracil derivatives have been reported to have antioxidative and anti-inflammatory activities (Goto et al., 2002; Isobe et al., 2003; Tobe et al., 2000). They are also known to be associated with coronary anticoagulant (South et al., 2006), antitumor (Yano et al., 2004; Pohlen et al., 2005), antiviral (Gazivoda et al., 2005), and herbicides activities (Kamireddy and Murray, 1996). In addition, some uracils are also used as efficient corrosion inhibitors (Dafali et al., 2003). On the other hand, nitrogen heterocyclic compounds have many applications in the rubber industry. For instance, 2mercaptobenzimidazle derivatives are known as rubber antioxidant and also to be useful to exhibit potent thyroid toxicity in rats (Sakemi et al., 2002). Raw rubber, either polar or nonpolar, has poor physicomechanical properties. To improve these properties, some ingredients such as accelerators, activators, antioxidants and softeners should be added to the raw rubber in small quantities, which could significantly affect the physical and mechanical properties of the mix. The current issue and full text archive of this journal is available at www.emeraldinsight.com/0369-9420.htm

Pigment & Resin Technology 36/4 (2007) 224– 234 q Emerald Group Publishing Limited [ISSN 0369-9420] [DOI 10.1108/03699420710761825]

224

New uracil derivatives as antioxidants for natural rubber

Pigment & Resin Technology

E.M.A. Yakout and S.H. El-Sabbagh

Volume 36 · Number 4 · 2007 · 224 –234

Experimental

Solvents and chemicals All solvents and chemical reagents were of pure grade and were further purified, if necessary, by distillation. Other rubber ingredients were those customarily used in rubber industries.

All melting points are uncorrected and were taken on an Electrothermal 9100 apparatus. IR spectra were recorded with a Carl Zeiss Spectrophotometer, model UR 10 in KBr pellets. The 1H- and 13C- NMR spectra were determined with a Varian Spectrometer at 200 MHz using TMS as an internal reference. Mass spectra were obtained using a Finigan SSQ 7000 mass spectrometer. Microanalysis was performed by the Microanalytical Laboratory, National Research Centre, Cairo. Compound 4 was prepared by a reported procedure (Allam et al., 2000).

Techniques for preparation and characterisation of rubber All rubber mixes were prepared on a laboratory two-roll mill of 470 mm diameter and 300 mm working distance. The speed of the slow roller was 24 rpm, with a 1:1.4 gear ratio. The rubber was mixed with other ingredients according to ASTM (D15-72) and careful control of temperature, nip gap and sequenced addition of ingredients. Vulcanization was carried out in a single-daylight electrically heated auto controlled hydraulic press at 142 ^ 18C and pressure 4 MPa. The compounded rubber and vulcanizates were tested according to standard methods, namely: . ASTM D2084-95 (1994) for determination of rheometric characteristic using a Monsanto Rheometer model 100. . ASTM D412-8a (1998) for determination of physicomechanical properties using Zwick tensile testing machine (model-1425). . Hardness was determined according to ASTM D 2240-97 (1997). . Tear resistance ASTM D 624(1998). . Fatigue properties were determined using a Monsanto Fatigue Failure Testing Machine, according to ASTM D 3629 (1998). . Swelling was determined according to ASTM D 47197(1998). It was possible to make use of the swelling data to calculate the molecular weight between two successive crosslinks (Mc) by the application of the well known Flory Rehner equation (Zhag et al., 2003).

Preparation of 1-(6-Amino-pyridine-2-yl)-5Cyano-1H-pyrimidine-2,4-Dione (ACD) (3) Compound 1 (0.01 mole, 2.1 g), triethylorthoformate (0.03 mole, 4.4 ml) and 2,6-diaminopyridine (0.01 mole) were refluxed in dioxane for 8 h (until the evolution of NH3 ceased). A precipitate was formed in the hot mixture, which was filtered off, dried and recrystallised from ethanol, m.p. 2258C; yield 82 per cent. 1 H NMR (D2O, DMSO-d6, TMS): d 6.20 – 7.73 (m, 5H, NH2 and 3H pyridine); d 8.85 (S, 1H, H-6, pyrimidine H); 10.89 (S, 1H, NH). 13 C NMR (DMSO-d6): d 88.35, 98.30, 98.75, 118.45, 139.84, 140.72, 147.0, 160.32, 160.65, and 164.93 (10 C, SP2 carbon atoms). IR (KBr): ncm2 1, 3,240, 3,200 (NH, NH2); 2,215 (CN); 1,730 (CO amide). MS: m/z (Mþ ¼ 229, 95 per cent). Anal. Calcd for C10H7N5O2 (229.20): C, 52.40; H, 3.08; N, 30.56 per cent. Found: C, 52.18; H, 2.95; N, 30.40 per cent. Preparation of 5-Benothiazol-2-yl-1-Hydroxy-1HPyrimidine-2,4-Dione (BHPD) (5) A suspension of compound 4 (0.01 mole, 2.35 g), triethylorthoformate (0.03 mole, 4.4 ml) and hydroxylamine hydrochloride (0.01 mole) in dioxane/dimethylformamide (40/10 ml) was refluxed for 8 h (the reaction was controlled by TLC). Upon completion of the reaction, the precipitate formed was collected from the hot reaction mixture, dried over suction and crystallised from DMF/H2O, m.p. . 3008C, yield 80 per cent. 1 H NMR (D2O, DMSO-d6, TMS): d 7.65 2 8.60 (m, 5H, 4 benzothiazole protons and 1H, pyrimidine H-6); 10.50 (s, 1H, NH); 3.5 (brs, 1H, OH, exchangeable with D2O). 13 C NMR (DMSO-d6): d 114.50, 124.75, 125.90, 127.25, 129.80, 136.05, 140.65, 147.10, 155.25, 158.15, 166.30 (11C, SP2 carbon atoms). IR (KBr): n cm2 1, 3,240 (OH); 3,200 (NH); 1,725 (CO amide). MS: m/z (Mþ ¼ 261, 85 per cent). Anal. Calcd for C11H7N3O3S (261.26): C, 50.57; H, 2.70; N, 16.08; S, 12.27 per cent. Found: C,50.32; H, 2.85; N, 15.81; S, 11.59 per cent.

Thus, cross-linking density (n) ¼ 1/(2MC): 1 21 lnð1 2 V R Þ þ V R þ mV R2 ¼ £ ð2McÞ 2r VR ðV R1=321=2 V R Þ

ð1Þ

where r is the density of rubber; V0 the molar volume of solvent absorbed (for toluene V0 ¼ 106.3 cm3/mole); VR is the volume fraction of the rubber in the swollen material (calculated as V R ¼ 1=ð1 þ Qm Þ, where Qm ¼ V =ðwd =rI Þ, V is volume of solvent absorbed by the rubber); and rI is the density of solvent and the interaction parameter constant (m) for NR is 0.393.

Results and discussion In the present work, we have found that cyano-acetyl urea (1), which was prepared at a high yield from low-cost and widely available chemicals (Bobranski and Synwiedski, 1948), could react with 2,6-diaminopyridine and triethylorthoformate in one pot reaction affording the valuable uracil derivative (3). It is assumed that, the active methylene group in (1) condenses with triethylorthoformate to form the ethoxy ylidene intermediate (2) which is attacked by the nitrogen

Materials Rubber Natural rubber, ribbed smoked sheets (RSS-1) with specific gravity 0.913, Mooney viscosity ML (1 þ 4) of 60-90 at 1008C and Tg at 758C. 225




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nucleophile via loss of ethanol with subsequent self-cyclisation and loss of ammonia to afford uracil derivative (3) (shown in Figure 1). On the other hand, the replacement of the cyano group in 1 by a heteroaryl moiety leads to produce heteroaryl analogue of 3. For example, N-(2-benzothiazolyl-acetyl) urea (4) reacts with triethylorthoformate and hydroxylamine hydrochloride as a nitrogen nucleophile to give benzothiazole uracil compound 5 at a good yield (Figure 2). The IR spectra of compounds 3 showed a characteristic absorption band at n ¼ 2,220 cm2 1 due to the presence of

cyano group, while the 1H NMR spectra of compounds 5 revealed H-6 pyrimidine proton as a singlet signal at d , 8.50 ppm. The structures of the new compounds were proven by spectral and analytical data (c.f. experimental). Effect of the compounds prepared on the vulcanization of NR mixe Two compounds namely 1-(6- Amino-pyridin-2-yl)-5-Cyano1H-pyrimidine-2,4-Dione (ACD) (3) and 5- Benothiazol-2yl-1-hydroxy-1H-Pyrmidine-2,4-Dione (BHPD) (5) were evaluated as antioxidants for NR.

Figure 1 NH2 H N

CH2 NC

NH2

C

C

O

O

N +

+

CH(OC2H5)3

H2 N 1

O

O

NC N H

NH2 –NH3

HC HN N

H2N 2

O NC NH

N

O

N

NH2 1-(6-Amino-pyridin-2-yl)-5-cyano-1H-pyrimidine-2,4-dione 3

226




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Figure 2 N

O

O NH2OH.HCl N H

S

NH2

CH(OC2H5)3

4

N O NH

S

N

O

OH

5-Benzothiazol-2-yl-1-hydroxy-1H-pyrimidine-2,4-dione 5

The uracial derivatives were incorporated into natural rubber mixes in equimolar quantities to phenyl-b-naphthyl amine (PbN) which is the antioxidant used widely in the rubber industry, this is due to their similar chemical structure. The rheometric characteristics, as well as the physico-mechanical properties of the vulcanizates, are given in Tables I and II. From data shown in Tables I and II, it is clearly seen that the

compounds prepared had increased scorch time, CRI, maximum and minimum torque while reduced optimum cure time. Also, clear are the increased tensile strength, modulus at 100 and 200 per cent strain, elongation at break (per cent) and decreased equilibrium swelling. In other words, these compounds have a secondary accelerating effect that can be explained by the fact that, these compounds are slightly basic. Consequently, when they are combined with the mercapto-benzothiazole disulphide (MBTs), which is slightly acidic, a weak synergistic effect results. Also, it is noticed that Dt (the difference between MH and ML) increase with these compounds, Dt may be taken as the extent of crosslinking in the rubber phase (Manna et al., 1999). It is

Table I Rubber formulations and rheometric characteristic of NR mixes Ingredient in phr

S0

NR 100 Stearic acid 2 Zinc oxide 5 40 Silitin Z a Processing oil 3 Sulphur 2.5 0.8 MBTS b PbN c – Prepared of compound 5 (BHPD) – 3 (ACD) – Rheometric characteristic at 142 6 18C Minimum torque (dN m) 3 Maximum torque (dN m) 34 Optimum cure time (min) 19 Scorch time (min) 3.5 Cure rate index (CRI) (min -1) 6.45 Dt(Mmax – Mmin) dN m 31

S1

S2

S3

100 2 5 40 3 2.5 0.8 1

100 2 5 40 3 2.5 0.8 –

100 2 5 40 3 2.5 0.8 –

– –

1.196 –

– 1.05

3.25 52 15 3.75 8.88 48.75

5 56 13.5 4.5 11.11 51

3.5 54 14.5 5 10.52 50.5

Table II Physico-mechanical properties of NR mixes containing the prepared compounds Ingredient in phr

S0

S1

S2

S3

s100 (MPA) 1.32 1.98 2.42 2.09 s200 (MPa) 2.54 2.7 3.1 2.75 Tensile strength (MPa) 10.79 11.2 13.07 14.6 Elongation at break (per cent) 630 680 714 719 Equilibrium swelling (per cent) 380 348 292 318 Soluble fraction, per cent 4.4 5.1 7.2 7 Molecular weight between crosslinks (Mc) (g/mole) 5,774 4,707 3,556 4,160 Crosslink density, n 3 10 4 (mole/cm3) 0.86 1.02 1.41 1.2 Tear resistance (N/cm) 21 25 50.5 46.6 Hardness (shore A) 44 50 51.3 60.2 No. of cycle until fracture (k cycle) 12.4 17.1 18.6 22.3

Notes: aSilitan Z (aluminum silicate); bMercapto-benzothiazole disulphide; c phenyl-b-naphthyl amine (PbN)

227




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evident that Dt is higher in case of compound 5 (BHPD) than compound 3 (ACD), i.e. the crosslink density increases for BHPD (mix No. S2).

Table IV Effect of different concentrations of compound ACD (3) on the rheometric characteristics and physico-mechanical properties of NR mixes Formulation No.

Effect of the different concentrations of the investigated compounds in natural rubber The selected compounds were incorporated in NR formulations at different concentrations as shown in Tables III and IV. Also, the rheometric characteristics of the mixes, as well as the physico-mechanical properties of the vulcanizates are presented in these tables. From the data shown in Table III, it is clear that the optimum concentration of BHPD as a secondary accelerator was achieved at 1.4 phr, as indicated by the highest cure rate index. It is also evident that, Dt has its highest value at these concentrations (max. No. S6, 1.4 phr). Also, one can see that all NR mixes with and without compounds under investigation have almost the same level of minimum torque (ML). It is worthy of notice that, BHPD increases the rate of vulcanization and decreases both the optimum cure time and scorch time more than ACD. This can be explained by its accelerating effect, which is attributed to the presence of the uracil-benzothiazol moiety. On the other hand, the NR vulcanizates containing 1.196 phr of the prepared compound BHPD (mix No. S2) exhibit greater tensile strength, strain at 100 per cent or 200 per cent stress, strain at break and tear strength, in comparison with the other mixes containing different concentrations of BHPD. Furthermore, these

S4

S5

S2

S6

S7

S10

S3

S11

S12

S13

Concentration in phr 0.5 0.8 1.05 1.4 1.7 2 Rheometric characteristics at 142 6 18C of compound 3 ML (dN m) 3 3.5 3.5 3.75 4 4 MH (dN m) 51 53 54 56 54 53 16 15 14.5 14 13 14 Tc90 (min) 6 5.75 5 4.5 4 3 Ts2 (min) 10 10.81 10.52 10.52 11.11 9.09 CRI, mi.2 1 Dt (M max-M min) (dN m) 48 49.5 50.5 52.25 50 49 Physic-mechanical properties of compound 3 Strain at 100 per cent stress (MPa) 1.6 1.8 2.09 1.7 1.58 1.5 Strain at 200 per cent stress (MPa) 2.72 2.78 2.75 2.73 2.69 2.68 Tensile strength (MPa) 12.39 13.5 14.6 13.2 13.1 12.8 Strain at break (per cent) 640 690 719 703 695 650 Equilibrium swelling (per cent) 335 325 318 322 326 324 Soluble fraction (per cent) 6.8 6.26 7 6.84 6.29 6.24 Tear strength (N/cm) 25 29 46.6 41.06 40 35 Hardness (shore A) 59.2 62.6 58.8 62.2 62.5 63 No. of cycle until fracture (k cycle) 15.9 16.8 22.3 28.93 29.04 33.4

formulations have the lowest value of equilibrium swelling and hardness. Also, from data in Table IV, it is clear that NR vulcanizates containing 1.05 phr of the prepared compound ACD (mix No. S3) have the highest values of the physical properties in comparison with the other mixes containing different concentrations of ACD. The results of hardness and fatigue life increase in the presence of prepared compound BHPD or ACD. This means that, physical properties were improved by using the investigated compounds. The stress-strain (s-1) curves for NR mixes without and with different concentrations of the investigated compounds are shown in Figures 3 and 4. It is clearly seen, from Figures 3 and 4 that, the mixes S2 and S3 exhibit the best properties (are the best formulations) in which 1.196 and 1.05 phr of BHPD and ACD are used. On the other hand, tensile properties of the elastomers were evaluated and an increase of the reduced stress [s *] are demonstrated (Priss, 1975) by equation (2):

Table III Effect of the different concentrations of compound BHPD (5) on the rheometric characteristics and physico-mechanical properties of NR mixes Formulation No.

S9

S8

Concentration in phr 0.5 0.8 1.196 1.4 1.7 2 Rheometric characteristics at 142 6 18C of compound 5 ML (dN m) 4.5 5.25 5 4.75 4.5 4 54 55 56 58 56.5 55 MH (dN m) Tc90 (min) 14.5 13.75 13.5 11 10.5 10 3.5 4 4.5 4.5 4 3.25 Ts2 (min) CRI (min 2 1) 9.09 10.26 11.11 15.38 15.38 14.81 Dt (M max-M min) (dN m) 49.5 49.75 51 53.25 52 51 Physic-mechanical properties of compound 5 Strain at 100 per cent stress (MPa) 1.5 1.7 2.42 1.99 1.88 1.8 Strain at 200 per cent stress (MPa) 2.8 2.92 3.1 3 2.72 2.7 Tensile strength (MPa) 11 11.6 13.2 12.8 12 11.8 Strain at break (per cent) 650 690 714 690 700 680 Equilibrium swelling (per cent) 298 308 292 305 302 301 Soluble fraction (per cent) 6.09 6.2 5.1 6.84 7.2 7.4 Tear strength N/cm) 40.5 46 50.5 45 39 30.6 Hardness (shore A) 58.8 59.5 51.3 60.75 63.2 68.2 No. of cycle until fracture (k cycle) 17.9 18.2 18.6 18.93 18.04 17.8

½s* ¼

s a 2 a22

ð2Þ

where s is the stress and a is the extension ratio. Also, tensile results are displayed in the form of the Mooney-Rivlin plots in Figures 5 and 6. The plots were based on the so-called Mooney-Rivlin equation (equation (3)): C1 þ C2 a 2 1 s¼2 a a2 228

ð3Þ




Volume 36 · Number 4 · 2007 · 224 –234

Figure 3 Stress – strain curvers for NR mixes without and with different concentrations of compound 5

Figure 5 Mooney – Rivlin plots for NR vulcanizates without and with different concentrations of compound 5 2

12

1.8 Reduced stress σ/2(α-α –2)

10

Stress, MPa

8 6 4 2

1.4 1.2 1 0.8 0.6 0.4 0.2 0

0 0

1

2 Strain

3

0

4

S0

S1

S2

S4

S5

S6

S7

S8

0.5

1

Reciprocial extension ratio 1/α S0 S4 S7

Figure 4 Stress – strain curvers for NR mixes without and with different concentrations of compound 3

S1 S5 S8

S2 S6

Figure 6 Mooney – Rivlin plots for NR vulcanizates without and with different concentrations of compound 3 2

7

1.8 Reduced stress σ/2(α-α –2),MPa

6 5 Stress, MPa

1.6

4 3 2 1

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2

0 0

1

2 Strain

3

S0

S1

S3

S9

S10

S11

S12

S13

0

4

0

where C1 is the term related to ideal elastic behaviour and C2 is a term expresses departures from ideal elastic behaviour. The constant C1 describes the behaviour predicted by the statistical theory of rubber-like elasticity and its value is directly proportional to the number of network chains per unit volume of the rubber vulcanizates. The value of C2 may be determined by the number of steric obstructions and the number of effectively trapped elastic entanglements as well as other network defects (Priss, 1975). Each of non-linear

0.5 Reciprocial extension ratio 1/α S0

S1

S3

S9

S10

S11

S12

S13

1

elasticity curves (Figures 5 and 6) shows the relation between the above mentioned function s=2ða 2 a22 Þ and 1/a over a range of low and medium strain. Owing to the pronounced curvature in the curves, the calculated of the constants C1 and C2 becomes difficult. By using strain-amplification factor x which takes into account both the disturbance of strain distribution, and the absence of deformation of fillers 229




Volume 36 · Number 4 · 2007 · 224 –234

(Kontou and Spathis, 1990), the curves in Figures 5 and 6 were shown in Figures 7 and 8. The strain-amplification factor x is given by equation (4):

than the overall strain. The extension ratio a in equation (3) should be recalculated from the measured overall strain by setting equation (5):

x¼

s ; 1

E0

E ¼ 1 þ 0:67fC þ 1:62f 2 C 2 E0

L ¼ 1 þ x1

ð4Þ

The use of the strain amplification factor x results in a linear portion of the Mooney-Rivlin plot for low and moderate strain by means of plot s=2ðL 2 L22 Þ versus L2 1.

where 1 is the strain produced by stress sf is the value of the shape factor and E0 is the modulus of each matrix greater

2

σ/2 (Λ-Λ–2), MPa

σ/2 (Λ-Λ–2), MPa

Figure 7 Stress – strain curves in Figure 5 re-plotted with the use of variable strain-amplification factor

1.5 1 0.5 0 0

0.5 1/Λ

2 1.5 1 0.5 0 0

1

0.5 1/Λ

S1 (b) 1 σ/2 (Λ-Λ–2), MPa

σ/2 (Λ-Λ–2), MPa

2 1.5 1 0.5 0 1 1/Λ

0.8 0.6 0.4 0.2 0

2

0

0.5 1/Λ

S4 (d) σ/2 (Λ-Λ–2), MPa

0.8 0.6 0.4 0.2 0 0.5 1/Λ

1 0.8 0.6 0.4 0.2 0

1

0

0.5 1/Λ

S6

1 S7

(e)

(f) 1

σ/2 (Λ-Λ–2), MPa

0

1 S5

(c) σ/2 (Λ-Λ–2), MPa

1 S2

(a)

0

ð5Þ

0.8 0.6 0.4 0.2 0 0

0.5 1/Λ

1 S8

(g)

230




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Figure 8 Stress – strain curves in Figure 6 re-plotted with the use of variable strain-amplification factor

1.4

0.8 0.7 0.6

σ/2 (Λ-Λ–2), MPa

σ/2 (Λ-Λ–2), MPa

1 0.9

0.5 0.4 0.3 0.2

1.2 1 0.8 0.6 0.4 0.2

0.1 0

0 0

0.5

0

1

1/Λ

0.5

1

1/Λ S3

S9

(a)

(b)

0.84

1

0.83

σ/2 (Λ-Λ–2), MPa

σ/2 (Λ-Λ–2), MPa

0.85 1.2

0.8 0.6 0.4 0.2

0.82 0.81 0.8 0.79 0.78 0.77

0

0.76 0

0.5

1

0

1/Λ

0.5

S10

S11

(c)

(d) 0.82

0.86

0.8

σ/2 (Λ-Λ–2), MPa

0.84

σ/2 (Λ-Λ–2), MPa

1

1/Λ

0.82 0.8 0.78 0.76 0.74

0.78 0.76 0.74 0.72 0.7

0.72 0.7

0.68 0

0.5

0

1

1/Λ

0.5

1

1/Λ S12

S13

(e)

(f)

From these plots, the constants C1 could be calculated from the intercept at L2 1 ¼ 0 and the slop gives the value of C2 (Kontou and Spathis, 1990). As discussed before, C1 describes the behaviour of the rubber vulcanizates and is a direct reflection of the number of network chains per unit volume, according to theories of rubber elasticity. Thus, the rubber vulcanizates containing the prepared compound BHPD (5) and ACD (3) have improved physical properties.

Effect of the compounds prepared on the thermal oxidative ageing of NR vulcanizates The NR mixes containing compounds BHPD and ACD (5 and 3) were subjected to thermal oxidative ageing in an aerated oven at 90 ^ 18C for different time periods up to seven days. The physico-mechanical properties of the aged samples were determined and the retained values of stress at rupture (tensile strength) and strain at break were calculated and are shown in Figures 9-11. From these figures, it is 231




Volume 36 · Number 4 · 2007 · 224 –234

clearly seen that the retained values of stress at rupture and strain at rupture decrease with the increase of the ageing time. On the other hand, one can see that the prepared compounds are more efficient antioxidants (S2 and S3) than the reference compound (S1). The efficiency of these prepared compounds greatly depends on the type of substitution groups in the uracil ring. These results are in good agreement with the retained values of equilibrium swelling which confirm that the prepared compounds protect the samples against oxidative degradation. The study of all curves of the relative changes in physicomechanical properties against time (Figures 9-11) showed that they obey the exponential relationship shown in equation (6):

values of each of the physico-mechanical properties with respect to ageing time (equation (7)):

Y ¼ aebt

ð2 # t # 7 daysÞ

dY ¼ abebt dt

ð7Þ

Figure 12(a)-(c) show the dependence of the rate of changes of physico-mechanical properties on the ageing time. It can be seen from these figures that the prepared compounds have higher antioxidants efficiency than the reference compound (PbN).

Conclusions

ð6Þ

It can be concluded that: . Compounds BHPD and ACD (5 and 3) achieved high performance as novel environmentally acceptable antioxidant or secondary accelerator for NR vulcanizates compared with PbN which is currently widely used in industries.

where Y is the relative change of the property, t is the ageing time in days, a and b are the constants calculated with a degree of fit $ 97 per cent. Differentiation of equation (6) with respect to time, gives the rate of change of the retained

120

100 80 60 40 20

100 80 60 40 20 0

0 0

5 Aging time, days S0 S2 S5 S7

0

10

5 Aging time, days S0 S2 S5 S7

S1 S4 S6 S8

120

R.V. of equilibrium swelling, %

120 R.V. of strain at break, %

Retained values of tensile strength, %

Figure 9 (a) The relation between aging time and retained values of tensile strength for compound 5; (b) the relation between aging time and retained values of strain or break; (c) the relation between aging time and retained values of equilibrium swelling

(a)

100 80 60 40 20 0 0

10

2 4 6 Aging time, days

S1 S4 S6 S8

S0 S2 S5 S7

(b)

8

S1 S4 S6 S8

(c)

120 100 80 60 40 20 0 0

2 4 6 Aging time, days S0 S3 S10 S12

(a)

S1 S9 S11 S13

8

120

120

100

100

R.V.of equilibrium swelling, %

R.V. of strain at rupture, %

R.V. of tensile strength, %

Figure 10 (a) Retained tensile strength vs aging time for NR vulcanizates containing concentrations of prepared compound 3; (b) retained strain at rupture vs aging time for NR vulcanzitaes containing different concentrations of the prepared compound 3; (c) retained equilibrium swelling in toluene vs aging time for NR vulcanizates containing different concentrations of prepared compound 3

80 60 40 20

80 60 40 20 0

0 0

2 4 6 Aging time, days S0 S3 S10 S12

(b)

232

S1 S9 S11 S13

8

0

2

4 6 Aging time, days S0 S3 S10 S12

(c)

S1 S9 S11 S13

8




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Figure 11 (a) Rate of change of tensile strength of NR vulcanizates containing the prepared compound 5 vs aging time; (b) rate of change of strain at rupture of NR vulcanizates containing the prepared compound 5 vs aging time; (c) rate of cange of equilibrium swelling on NR vulcanizates containing the prepared compound 5 0 0

2

4

6

0

8

–1

–4

dc /dt

dc /dt

–2

–6 –8

0

2

4

6

8

–2 –3 –4

–10

–5 Aging time, days

Aging time, days

S0

S1

S2

S4

S5

S6

S7

S0 S4 S7

S8

S1 S5 S8

(a)

S2 S6

(b)

0 dc / dt

–2

0

2

4

6

8

–4 –6 –8 Aging time, days S0 S4

S1 S5

S7

S2 S6

S8 (c)

Figure 12 (a) rate of change of tensile strength of NR vulcaniaztes containing the prepared compound 3; (b) rate of change of strain at rupture of NR vulcanizates containing the prepared compound 3 vs aging time; (c) rate of change of equilibrium swelling of NR vulcanizates containing the prepared compound 3 0

0

0 2

4

6

8

–1

–4 –6

0

5

10

–1

–3

–8

–4

–10

–5

–12


0

2

4

6

8

–2

–2

dc / dt

0

dc /dt

dc / dt

–2

–3 –4 –5 –6

Aging time, days


S0 S3 S10 S12

S1 S9 S11 S13

S0 S3 S10 S12

(a) .

.

.

S1 S9 S11 S13

(b)

S0 S3 S10 S12

S1 S9 S11 S13

(c)

References

The optimum concentrations of compounds BHPD and ACD (5 and 3) for NR vulcanizates as secondary accelerator and also antioxidant are 1.196 and 1.05 phr, respectively. Also, it is clear that compounds BHPD and ACD (5 and 3) can protect NR vulcanizates against thermal ageing. These results can be attributed to the presence of the uracil-benzothiazol moeity in compound BHPD (5) and pyridine moity in compound ACD (3). When compound 5 or 3 was added to NR vulcanizates, they decreased the equilibrium swelling while improved the tensile and tear strength, hardness and fatigue life.

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About the authors E.M.A. Yakout is an Assistant Professor of Organic Chemistry in Chemistry of Pesticides Department at National Research Centre, Dokki, Cario, Egypt. He graduated from Faculty of Science, Al-Azhar University with a BSc in Chemistry in 1976, an MSc from Ain Shams University in Chemistry in 1984, and a PhD from Cairo University in Organic Chemistry in 1989. His current research interest covers synthesis and characterisation of new heterocyclic compounds with anticipated industrial applications. S.H. El-Sabbagh graduated from Ain Shams University with a BSc in Physics in 1981 and was awarded an MSc and a PhD in Physical Properties of Rubber at Cairo University in 1991 and 1995, respectively. El-Sabbagh is currently at the Chemical Industries Research Division, National Research Centre, Cairo. El-Sabbagh’s research interest covers the physical properties of rubber and plastics technology. S.H. El-Sabbagh is a member of several scientific societies. S.H. El-Sabbagh is the corresponding author and can be contacted at: [email protected]

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