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Role of dichlorocarbene modified styrene butadiene rubber in compatibilisation of styrene butadiene rubber and chloroprene rubber blends
European Polymer Journal 37 (2001) 719±728
Role of dichlorocarbene modi®ed styrene butadiene rubber in compatibilisation of styrene butadiene rubber and chloroprene rubber blends M.T. Ramesan a, George Mathew b, Baby Kuriakose a, Rosamma Alex a,* a
Rubber Research Institute of India, Kottayam-9, Kerala, India b CMS College, Kottayam-1, Kerala, India Received 28 January 2000; accepted 13 June 2000
Abstract A study was conducted on the use of dichlorocarbene modi®ed styrene butadiene rubber (DCSBR) for the compatibilisation of blends of styrene butadiene rubber (SBR) and chloroprene rubber (CR). Stress±strain behaviour, mechanical properties, and low temperature transitions of the blends were examined in order to elucidate the eciency of the compatibiliser. It was found that eective compatibilisation was achieved when chlorine content of DCSBR was 25% and its dosage was 5 phr (parts per hundred rubber). Thermal analysis by DSC showed that an appreciable extent of molecular level miscibility has been achieved in SBR/CR blends by using DCSBR as a compatibiliser. The formation of interfacial crosslinks in the presence of DCSBR is evident from the cure characteristics and stress±strain isotherms. An increase in tensile strength, tear strength, resilience and hardness and a decrease in compression set was observed when the chlorine content and dosage of the compatibiliser increased from 15% to 25% and 5 phr, respectively. Compatibilised blends showed enhanced mechanical properties in the presence of reinforcing ®llers such as HAF carbon black and precipitated silica. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Compatibiliser; Dichlorocarbene modi®ed styrene butadiene rubber; Styrene butadiene rubber/chloroprene rubber blend
1. Introduction Blending of elastomers is being practised to achieve the best balance of physical properties, processability and cost. Elastomeric blends can either be miscible or immiscible depending on the free energy of mixing, but the number of miscible elastomer blends has been very few [1]. Blending of two incompatible polymers yields a material with poor mechanical properties. However, dierent studies have demonstrated that the mechanical properties of such blends can be signi®cantly improved by adding a suitable compatibiliser [2±6] during processing. These materials decrease the macroscopic in-
*
Corresponding author. Fax: +91-481-578-317.
homogeneities and increase the stability of the morphology during processing by reducing the interfacial tension and subsequently the dispersed phase size [7±10], as expected from Taylor's theory [11]. Speci®c interaction of polymer with functional groups can increase the extent of thermodynamic compatibility. Compatibility of polymer blends can also be improved by the use of copolymers containing segments chemically identical to the individual homopolymers [12±15]. If one of the polymers of a graft copolymer is miscible with one of the phases, then also the blend can be compatibilised [16]. Thermodynamic theories concerning the emulsifying eect of block copolymers in polymer blends have been developed by Leibler [17,18], and Noolandi and Hong [19±21]. Blends of SBR and BR were found to be more compatible after vulcanisation by Livingston et al. [22].
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 1 5 7 - 9
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Blends based on functionally active rubbers are reported to be miscible due to crosslinking reaction by the functional groups [23±25]. Chlorinated polyethylene acts as a compatibiliser in chloroprene rubber (CR)/ethylene propylene diene monomer (EPDM) rubber and acrylonitrile butadiene (NBR)/EPDM blends [26]. Polymethyl methacrylate (PMMA) acts as a compatibiliser in blends of NR/NBR [27]. CR is also a compatibiliser in NR/ NBR blends [27]. Blending of SBR with CR has been done to obtain better crystallisation resistance, better compression set resistance, lower brittleness temperature and enhanced resistance to sunlight deterioration as compared with CR alone. Other important properties, such as oil, heat, ¯ame and ozone resistance, decrease as the amount of SBR increases. It is possible to improve the phase morphology of SBR/CR blend by incorporating 5±10 phr of a modi®ed copolymer that has segments chemically identical to SBR and CR phases. Addition of halocarbenes to polymers has been reported since a long time; however, available information on the commercial application on such modi®ed polymers is limited. In this paper, eciency of dichlorocarbene modi®ed SBR as a compatibiliser in SBR/CR blends is reported. The eects of the level of modi®cations and the amount of modi®ed SBR required for compatibilisation are studied in detail. It is considered that there would be sucient interaction between the polar segments of modi®ed SBR with CR phase and butadiene part with SBR phase, thereby reducing the interfacial tension. 2. Experimental 2.1. Materials Styrene butadiene rubber (Synaprene 1502) having 25% bound styrene was obtained from Synthetics and Chemicals Ltd., Bareilly, UP, India. CR (Neoprene W type) having 36% chlorine content was obtained from Du Pont Co., USA. Dichlorocarbene modi®ed SBR (DCSBR) used as the compatibiliser was prepared by the reaction of dichlorocarbene produced by the alkaline hydrolysis of chloroform with SBR using phase transfer catalyst as reported earlier [28]. The level of modi®cation was monitored by the determination of chlorine content using chemical analysis techniques. The structural changes taking place during chemical modi®cation is shown in Scheme 1. Master batches of SBR and CR were prepared separately and then blended on a laboratory size two roll mixing mill (15 30 cm2 ) at a friction ratio 1:1.25 as per ASTM D-15-627. The temperature of the mill was maintained at 30°C by the circulation of cold water through the rolls. The compatibiliser of varying chlorine contents from 15% to 35% was added to preblended
Scheme 1. Structural changes of SBR during dichlorocarbene modi®cation.
SBR/CR (70/30, 50/50 and 30/70) at a constant dosage of 5 phr. The compatibiliser having 25% chlorine content was added at various dosages of 1±10 phr to 50/50 SBR/CR preblend compositions. Filled compounds were prepared by adding HAF (high abrasion furnace) carbon black and precipitated silica to the 50/50 blend compatibilised by 5 phr DCSBR containing 25% chlorine. The basic formulations used in the study are presented in Tables 1±3. Monsanto rheometric studies of the mixes (Fig. 1) were carried out at 150°C with 3° arc of rotor oscillation according to ASTM D-2705. The samples were vulcanised to their respective optimum cure time in a hydraulic press at 150°C and a pressure of 45 kg cmÿ2 on the mould. The tensile strength and tear resistance of the samples were tested with a Zwick universal testing machine (model-1474) at a temperature of 25 2°C and at a crosshead speed of 500 mm minÿ1 , according to ASTM D-412-80 and ASTM D-624-81, respectively. All other Table 1 Formulation of SBR/CR blends in the presence and absence of DCSBR Chemical
S7
S5
S3
S7
S5
S3
SBR CR DCSBR Zinc oxide Magnesium oxide Stearic acid CBSa TMTDb NA22c TDQd Sulphur
70 30 ± 5 1.2
50 50 ± 5 5
30 70 ± 5 2.8
70 30 5 5 1.2
50 50 5 5 2
30 70 5 5 2.8
1.15 0.7 0.15 0.15 0.7 1.54
1.0 0.5 0.25 0.25 0.5 1.1
0.8 0.3 0.35 0.35 0.3 0.66
1.15 0.7 0.15 0.15 0.7 1.54
1.0 0.5 0.25 0.25 0.5 1.1
0.8 0.3 0.35 0.35 0.3 0.66
*
DCSBR containing (a) 15%, (b) 20%, (c) 25%, (d) 30% and (e) 35% of chlorine content, respectively, were added to these blends. a Cyclohexyl benzathiazole. b Tetramethyl thiuram disulphide. c Ethylene thiourea. d 2,2,4-Trimethyl 1,2-dihydroquinoline.
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Table 2 Formulation of 50/50 SBR/CR carbon black ®lled blends in the presence and absence of DCSBR Chemical
CU1
CC1
CU2
CC2
CU3
CC3
CU4
CC4
SBR CR DCSBRa Zinc oxide Magnesium oxide Stearic acid CBS TMTD NA22 TDQ Aromatic oil HAF C-black Sulphur
50 50 ± 5 2 1 5 0.25 0.25 1.0 0.7 10 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 0.7 10 1.1
50 50 ± 5 2 1 5 0.25 0.25 1.0 1.4 20 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 1.4 20 1.1
50 50 ± 5 2 1 5 0.25 0.25 1.0 2.1 30 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 2.1 30 1.1
50 50 ± 5 2 1 5 0.25 0.25 1.0 2.8 40 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 2.8 40 1.1
a
DCSBR containing 25% of chlorine content.
Table 3 Formulation of 50/50 SBR/CR precipitated silica ®lled blends in the presence and absence of DCSBR Chemical
SU1
SC1
SU2
SC2
SU3
SC3
SU4
SC4
SBR CR DCSBRa Zinc oxide Magnesium oxide Stearic acid CBS TMTD NA22 TDQ Aromatic oil Precipitated silica Diethylene glycol Sulphur
50 50 ± 5 2 1 5 0.25 0.25 1.0 0.7 10 0.5 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 0.7 10 0.5 1.1
50 50 ± 5 2 1 5 0.25 0.25 1.0 1.4 20 0.5 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 1.4 20 0.5 1.1
50 50 ± 5 2 1 5 0.25 0.25 1.0 2.1 30 1 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 2.1 30 1 1.1
50 50 ± 5 2 1 5 0.25 0.25 1.0 2.8 40 1.5 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 2.8 40 1.5 1.1
a
DCSBR containing 25% of chlorine content.
technical properties were determined as per the relevant ASTM standards. The eciency of the compatibiliser was assessed by determination of Tg using a Perkin±Elmer dierential scanning calorimeter, operated at a heating rate of 15°C minÿ1 within the temperature range of ÿ80°C to 25°C.
3. Results and discussion
ÿ33°C. For the uncompatibilised blend, there appears to be two transitions in the temperature range from ÿ60°C to ÿ30°C which indicates the presence of microlevel inhomogeneity. The transitions at ÿ56°C and ÿ33°C for the blends correspond to the transitions of pure SBR and pure CR, respectively. However, the blend with 5 phr containing 25% chlorine and DCSBR shows a single transition in the temperature range ÿ55°C to ÿ33°C with a midpoint around ÿ40°C. This shows that DCSBR acts as a compatibiliser in blends of SBR/CR.
3.1. Thermal analysis
3.2. Processing characteristics from rheometric data
The DSC traces of pure SBR and CR and their 50/50 blend in the presence and absence of compatibiliser are shown in Fig. 1. Pure SBR shows a glass transition temperature (Tg ) at ÿ56°C and pure CR shows a Tg at
The maximum torque, optimum cure time, and scorch time change signi®cantly with blend composition as given in Table 4 and shown by the rheographs of the mixes in Fig. 2a±d. In all blend compositions, a
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crosslink density and chain entanglement. As a result of the comparatively high molecular weight of SBR, the long chain tends to coil. The dierent portions of the chain entangle with chain of CR and its own chains. High extent of entanglement causes an increase in the maximum torque value and hence the Mh-Mn value. The higher Mh-Mn value of the 70/30 SBR/CR blend compared to the 50/50 blend can also be due to a higher crosslink density achieved in 70/30 SBR/CR blend through better distribution of the curatives. There is a progressive increase in scorch time as SBR concentration in the blend increases. Crosslinking of CR in the presence of ZnO and accelerators such as TMTD and ETU proceed very rapidly, and ZnO or ZnCl2 formed catalyses the reaction [29]. The mechanism of crosslinking by sulphenamide accelerator in SBR is accompanied by a great processing safety. With addition of 5 phr DCSBR of chlorine content varying from 15% to 35%, the maximum torque attained increases with a decrease in scorch time in all blend compositions showing that there is a greater crosslinking. The presence of DCSBR brings about sucient adhesion between the phases [30] due to the polar interaction among blend constituents. Thus, there is a reduction of surface energy mismatch between polymers, and there is sucient interfacial crosslinks. This interfacial crosslink formation increases as the concentration of chlorine in DCSBR increases.
Fig. 1. DSC thermograms of 50/50 SBR/CR blend: (a) without compatibiliser and (b) compatibiliser with 5 phr loading.
comparatively high optimum torque is obtained when the concentration of CR in the blend is high. It is already reported that the maximum torque is dependent on Table 4 Cure characteristics of SBR/CR blends compatibilised by DCSBR Blend composition
Chlorine content of DCSBR (%)
Optimum cure time at 150°C (t90 ) (min)
SBR/CR 70/30
0 15 20 25 30 35
26.5 28 29 30 26 25
5.5 5 4.75 4.5 4.25 4
30 32 33 34 34 33
SBR/CR 50/50
0 15 20 25 30 35
22 24 25.5 27 25 24
4 3.75 3.5 3.25 3 2.8
28 29 30 32 31 30
Dosage of compatibiliser (phr) having 25% chlorine content in 50/50 blend
1 3 5 10
23 25 27 25
3.75 3.5 3.25 3
29 30 32 29
SBR/CR 30/70
0 15 20 25 30 35
16 18.5 20 21 18 16
3 2.75 2.5 2 1.75 1.5
31 33 34 36 35 35
Scorch time at 150°C (t2 ) (min)
Mh-Mn (dN m)
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Fig. 2. Rheographs of (a) 70/30 SBR/CR blend and compatibilised blend containing 5 phr DCSBR of varying chlorine contents at 150°C: a. 15%, b. 20%, c. 25%, d. 30% and e. 35%; (b) 50/50 SBR/CR blend and compatibilised blend containing 5 phr DCSBR of varying chlorine content at 150°C: a. 15%, b. 20%, c. 25%, d. 30% and e. 35%; (c) 30/70 SBR/CR blend and compatibilised blend containing 5 phr DCSBR of varying chlorine content at 150°C: a. 15%, b. 20%, c. 25%, d. 30% and e. 35%; (d) 50/50 SBR/CR blend and compatibilised blend containing dierent dosage of DCSBR (25%) at 150°C: a. 0 phr, b. 1 phr, c. 3 phr, d. 5 phr and e. 10 phr.
As the concentration of SBR in the blend increases, the cure characteristics such as optimum cure time, scorch time and induction time at 150°C increases for the uncompatibilised blends. The mechanism of vulcanisation is dierent in the two rubbers, and polychloroprene compounds are known to be scorchy. Since the amount of accelerator is ®xed in these compositions, the curing properties suer an increase. The higher scorch time of 70/30 SBR/CR blend reveals its comparatively better scorch safety. There is an increase in optimum cure time as chlorine content of DCSBR increases up to 25%, and beyond that level of chlorination, the optimum cure time decreases. When the chlorine content is 25% and the loading is 5 phr, the compatibilising eciency is maximum (as evidenced from tensile property). This and compatibilisation can be due to microlevel interactions large interfacial area formed by the uniformly distributed microdomains. Since more reactants are needed for
the formation of a greater number of interface crosslinks, the optimum cure time is found to increase. But as the chlorine content increases further, the compatibiliser eciency seems to decrease, which ultimately ends or results in the coalescence of dispersed domains. This reduces the interfacial area and thereby causes a decrease in optimum cure time. The eect of dosage of compatibiliser on rheometric torque is shown in Fig. 2d. The maximum torque attained increases with the loading of the compatibiliser. However, at a dosage of 10 phr, the maximum torque is only marginally higher than that obtained with 5 phr DCSBR. It is clear that as the loading of the compatibiliser increases, there is greater interaction among the constituents promoting interfacial crosslinks. The optimum cure time increases as the dosage of DCSBR increases up to 5 phr, and beyond this dosage, the value drops. Hence, optimum vulcanisation characteristics are obtained with DCSBR of chlorine content 25% and at a dosage of 5 phr.
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3.3. Calculation of crosslink density 3.3.1. Stress±strain isotherms The extent of physical crosslinks in an elastomer vulcanisate can be assessed by using Mooney Rivlin equation [31±33]. The equation is given by F 2A0 k ÿ kÿ1 C1 kÿ1 C2 ;
1
where F is the extension force required to stretch a piece of rubber vulcanisate (dumb-bell) of cross-section area, A0 to an extension ratio, k. A plot of F =2A0 k ÿ kÿ1 vs. kÿ1 gives a straight line whose y intercept C1 is directly related to the physically eective crosslink density, mphys , by the equation C1 qRT mphys :
2
The plots obtained are presented in Fig. 3, and the values of crosslink density are shown in Table 5. It can be seen that the force and crosslink density increases as the concentration of DCSBR increases in 50/50 blend and is in agreement with the increase in rheometric torque. C2 , a term which serves as a measure of departure of observed stress±strain relationship from the form suggested by statistical theories [34], is given in Table 5. 2C2 value increases with the loading of compatibiliser up to 5 phr and then decreases. The observation of a higher 2C2 for the 50/50 blend containing 5 phr DCSBR shows the presence of higher chain entanglement [34]. Since the elastomer matrix is composed of two components, a higher entanglement shows better molecular level mixing. At higher concentrations of DCSBR, there is probably less chain entanglement and hence molecular level mixing though there is a much enhanced polar interaction. Here, it acts as a third component. 3.3.2. Swelling studies Equilibrium swelling in toluene to assess the volume fraction Vr of the rubber network in the swollen gel was calculated by the method reported by Ellis and Welding [35] and is presented in Table 5.
Fig. 3. Plots of F =2A0 k ÿ kÿ1 vs. kÿ1 of 50/50 SBR/CR blend with dierent dosages of compatibiliser.
Vr
d ÿ fwqÿ1 r ; ÿ1 d ÿ fw qÿ1 r A0 qs
3
where d is the weight after drying out the sample, and w is the weight of swollen sample. A0 is the weight of the absorbed solvent, f, the fraction of insoluble components, qr and qs , the densities of the rubber and solvent, respectively. Vr increases with the addition of compatibiliser progressively up to 5 phr and at still higher concentrations, the value of Vr drops. This is due to the decrease in the extent of swelling caused by higher polymer±polymer interactions and increased interfacial crosslinks formed in the presence of compatibiliser. This is in good agreement with the crosslink density obtained from the stress±strain data. 3.4. Eect of compatibiliser on technological properties The physical properties of the blends are given in Tables 6±8. In pure blends, tensile strength, elongation at break and modulus increase as CR concentration
Table 5 Crosslink density parameters C1 , C2 , mphys , Vr and dierence in rheometric torque of 50/50 SBR/CR blend with varying concentration of compatibiliser Compatibiliser loading (phr)
mphys 10ÿ3 (g mol mlÿ1 )
Mh-Mn (dN m)
2C2 (N mmÿ2 )
2C1 (N mmÿ2 )
Vr (from swelling studies)
0 1 3 5 10
1.06 1.22 1.98 2.75 2.09
28 29 30 32 29
1.04 2.32 3.36 4.0 3.12
0.536 0.686 0.750 0.890 0.82
0.3364 0.3568 0.3685 0.3874 0.3766
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Table 6 Mechanical properties of blends of 70/30 SBR/CR compatibilised by DCSBR of chlorine content varying from 15% to 35% Properties
Percentage of chlorine in DCSBR used at 5 phr loading (1)
(2)
(3)
(4)
(5)
(6)
Modulus, 300% (MPa) Tensile strength (MPa) Elongation at break (%) Tear strength (kN mÿ1 ) Hardness (Shore A) Resilience (%) (Dunlop tripsometer) Compression set (%) 22 h at 70°C
1.7 3.2 430 19.4 31 62
1.8 3.3 370 16.6 33 63
1.82 3.4 380 18.8 35 64
1.67 3.8 500 19.1 36 65
1.83 3.5 420 19.8 38 64
1.85 3.19 390 19.4 39 63
18.6
17.5
16.6
15.4
15.8
16.9
In (2)±(6), the DCSBR blends contain 15%, 20%, 25%, 30% and 35% of chlorine content, respectively.
Table 7 Mechanical properties of blends of 50/50 SBR/CR compatibilised by DCSBR of chlorine content varying from 15% to 35% Properties
Percentage of chlorine in DCSBR used at 5 phr loading (1)
(2)
(3)
(4)
(5)
(6)
Modulus, 300% (MPa) Tensile strength (MPa) Elongation at break (%) Tear strength (kN mÿ1 ) Hardness (Shore A) Resilience (%) (Dunlop tripsometer) Compression set (%) 22 h at 70°C
1.6 3.9 520 16.7 33 60
1.7 4.6 550 16.0 33 61
2.1 5.73 580 19.1 36 63
2.94 6.5 680 20.9 39 64
2.48 5.8 600 20.4 41 62
2.0 5.2 490 19.0 43 61
20.8
20.8
19.4
18.1
20.2
22.0
In (2)±(6), the DCSBR blends contain 15%, 20%, 25%, 30% and 35% of chlorine content, respectively.
Table 8 Mechanical properties of blends of 30/70 SBR/CR compatibilised by DCSBR of chlorine content varying from 15% to 35% Properties
Percentage of chlorine in DCSBR used at 5 phr loading (1)
(2)
(3)
(4)
(5)
(6)
Modulus, 300% (MPa) Tensile strength (MPa) Elongation at break (%) Tear strength (kN mÿ1 ) Hardness (Shore A) Resilience (%) (Dunlop tripsometer) Compression set (%) 22 h at 70°C
2.39 8.10 690 25 28 53 32.5
1.71 6.9 650 20.5 31 54 31.6
1.31 7.0 750 21.6 33 54 28.8
1.80 8.4 800 25.8 35 55 25.8
1.28 6.4 750 21.7 37 52 27.8
1.65 6.6 700 18.9 38 51 28
In (2)±(6), the DCSBR blends contain 15%, 20%, 25%, 30% and 35% of chlorine content, respectively.
increases. The resilience shows a decrease, whereas compression set increases with the increase in CR concentration in the blends. Mechanical properties in all blend compositions improve with addition of 5 phr DCSBR of chlorine content varying from 15% to 35%. The level of improvement depends on the chlorine content of DCSBR. Tensile strength and elongation at
break, hardness and resilience increase while compression set decreases with an increase in chlorine content of DCSBR up to 25% and then decreases. Ecient compatibilisation by 25% chlorine containing DCSBR creates strong interfacial crosslinks which result in an increase in tensile strength. The improvement in mechanical properties is more eective at 50/50 blend
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Table 9 Mechanical properties of blends of 50/50 SBR/CR compatibilised by DCSBR Properties
Loading of 25% chlorine containing DCSBR (phr) 0
1
3
5
10
Modulus, 300% (MPa) Tensile strength (MPa) Elongation at break (%) Tear strength (kN mÿ1 ) Hardness (Shore A) Resilience (%) (Dunlop tripsometer) Compression set (%) 22 h at 70°C
1.66 3.9 520 16.7 33 60 20.8
1.98 4.6 400 17 34 62 19.3
2.35 5.73 440 18.4 37 63 18.8
2.94 6.5 680 20.9 39 64 18.1
2.8 5.8 570 21.3 42 61 21.3
composition. A comparatively higher tensile strength, elongation at break and tear strength is obtained for the compatibilised 30/70 SBR/CR blend. The microscopic extension ratio of the dispersed CR domains is much higher than the macroscopic extension ratio during tensile test. Greater extension ratios cause strain induced crystallisation in CR which results better tensile properties in these cases. Owing to the absence of ecient interfacial crosslink, the uncompatibilised blend shows comparatively poor mechanical properties. The tensile strength values for compatibilised SBR/CR 70/ 30 are found to comparatively lower due to higher proportions of amorphous and weak SBR in this blend . The tensile strength of 50/50 composition increases with the loading of compatibiliser (Table 9) up to 5 phr, and with further increase in the concentration of compatibiliser, tensile strength does not change much. This may be due to the supersaturation of the interphase with the compatibiliser which increases interfacial tension.
Fig. 4. Rheographs at 150°C of carbon black ®lled 50/50 SBR/ CR blend and compatibilised blend.
3.5. Eect of dierent ®llers on compatibilised SBR/CR (50/50) blends The rheographs of carbon black and silica ®lled blends in the presence and absence of compatibiliser are presented in Figs. 4 and 5. The rheometric torque increases with the addition of 5 phr of DCSBR. The optimum cure time increases, whereas scorch time decreases with the addition of ®ller. The carbon black ®lled blends shows a comparatively higher Mh-Mn value, higher optimum cure time and lower scorch time as compared to silica ®lled blends. There is enhancement in technical properties with the addition of ®ller as shown in Tables 10 and 11. The blend containing ®llers of carbon black and silica shows a higher tensile strength, tear strength and abrasion resistance as compared to uncompatibilised blends. This is due to the improvement in polymer ®ller interaction resulting from the eective interfacial adhesion and the uniform distribution of the phases.
Fig. 5. Rheographs at 150°C of silica ®lled 50/50 SBR/CR blend and compatibilised blend.
The heat build-up and compression set values decreases whereas resilience increases on compatibilisation. The compatibiliser present in the blend reduces the heat build-up due to the formation of eective interface which avoids severe friction between component and polymer ®ller friction. The heat build-up increases with the increase in ®ller loading, and the eect is maximum for the carbon black ®lled samples.
M.T. Ramesan et al. / European Polymer Journal 37 (2001) 719±728
727
Table 10 Mechanical properties of compatibilised 50/50 SBR/CR blend with dierent loading of carbon black ®ller Properties
CU1
CC1
CU2
CC2
CU3
CC3
CU4
CC4
Modulus, 300% (MPa) Tensil strength (MPa) Elongation at break (%) Tear strength (kN mÿ1 ) Din abrasion loss (mm3 ) Heat build-up (°C) Compression set (%) 22 h at 70°C Resilience (%) (Dunlop tripsometer)
3.2 8.0 560 31.3 196 31 16.8 60
4.1 9.5 540 41.0 162 28 15.2 60
3.9 11.8 530 46.0 129 37 15.4 52
4.8 14.8 510 50.6 105 33 13.0 58
5.3 16.5 500 55.1 111 42 13.4 49
6.1 19.0 475 57.1 93 36 11.2 55
5.8 19.8 560 56.2 96 47 12.4 41
6.7 21.2 430 62.0 70 43 10.9 50
Table 11 Mechanical properties of compatibilised 50/50 SBR/CR blend with dierent loading of silica ®ller Properties
SU1
SC1
SU2
SC2
SU3
SC3
SU4
SC4
Modulus, 300% (MPa) Tensil strength (MPa) Elongation at break (%) Tear strength (kN mÿ1 ) Din abrasion loss (mm3 ) Heat build-up (°C) Compression set (%) 22 h at 70°C Resilience (%) (Dunlop tripsometer)
2.5 5.8 580 23.7 254 29 24 64
3.0 7.8 570 25.9 216 25 21 65
2.9 10.1 550 25.5 181 32 21 62
3.7 12.3 530 28.4 150 28 19 64
3.5 11.2 540 30.6 132 36 20 57
4.1 13.0 490 32.8 119 32 18 62
4.0 14.1 470 35.4 114 38 18 52
4.9 15.0 455 37.8 109 34 16 60
4. Conclusion DCSBR acts as a compatibiliser in the otherwise incompatible blends of SBR/CR. The compatibilisation is accompanied by an improvement in mechanical properties and depends on chlorine content and concentration of DCSBR. The compatibilising action is more ecient in 50/50 blend and increases progressively with chlorine content of compatibiliser up to 25% and a dosage up to 5 phr, beyond which there is no further increment. The presence of chain entanglements as revealed from stress±strain isotherm is due to better molecular level mixing and leads to enhanced physical properties. The incorporation of ®ller imparts enhanced the physical properties in compatibilised SBR/CR blends. References [1] Roland CM. In: Bhowmick AK, Stephens HL, editors. Handbook of elastomers new developments technology. New York: Marcel Dekker, 1988. [2] Hess WM, Scott CE, Callan JE. Rubber Chem Technol 1967;40:371±82. [3] Marsh PA, Voet A, Price LD. Rubber Chem Technol 1967;40:359±63. [4] Fayt R, Jerome R, Teyssie PH. J Polym Sci Part B Polym Phys 1982;20:2209±17. [5] Joseph A, George KE. Kunststoe 1991;44:538±41.
[6] Barentsen WM, Heikens D. Polymer 1973;14:579±85. [7] Teyssie PH, Fayt R, Jerome R. J Polym Sci Part B: Polym Phys 1981;19:1269±72. [8] Barentsen W, Heikens D, Piet P. Polymer 1974;15:119±26. [9] Heikens D, Barentsen WM. Polymer 1977;18:69±72. [10] Fayt R, Jerome R, Teyssie Ph. Makromol Chem 1986; 187:837±52. [11] Taylor GI, Pole R. Soc London 1934;A146:501±23. [12] Setua DK, White JL. Kautscheuk Gummi Kunststoe 1991;44(9):821±6. [13] Gaylord NG. In: Platzer AJ, editor. Copolymers polymer blends composites. ACS: Washington DC. [14] Locke CE, Paul DR. J Appl Polym Sci 1973;17:2597±604. [15] Ide F, Hakgawer A. J Appl Polym Sci 1979;18:963±72. [16] Anastasiadas SH, Gancarz I, Koberstein JT. Macromolecules 1989;22:1449±55. [17] Leibler L. Macromol Chem Makromol Symp 1985;16: 17±23. [18] Leibler L. Macromolecules 1982;15:1283±9. [19] Noolandi J. Polym Engng Sci 1984;24:70±8. [20] Noolandi J, Hong KM. Macromolecules 1982;15:482±6. [21] Noolandi J, Hong KK. Macromolecules 1984;17:1531±5. [22] Livingston DI, Rongone RL. Proc Int Rubber Conf, Brighton, UK, no. 22, 1967. [23] Dunnon DD, de Decker HK. Rubber Age 1965;97(8): 85±93. [24] Fujimoto K, Yoshimiya N. Rubber Chem Technol 1968; 41:699±709. [25] Marsh PA, Voet A, Price LD, Mullens TJ. Rubber Chem Technol 1968;41:344±58. [26] Setua DK, White JL. Kautschuk Gummi Kunststoe 1991;44(6):542±7.
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[27] Tinker AJ. Blends of natural rubber with speciality synthetic rubbers in industrial composites based on natural rubber. Malaysial Rubber Research Development Board, Kulalumpur, p. 103±13. [28] Ramesan MT, Rosamma A. J Appl Polym Sci 1998; 68:153±60. [29] Miyata AM. Rubber Chem Technol 1989;62:1±11. [30] Coran AY. Rubber Chem Technol 1988;61:281±94.
[31] Treloar LRG. The physics of rubber elasticity. 3rd ed. Oxford: Clarendon Press; 1976. [32] Rivlin RS, Saunders DW. Phil Trans Roy Soc Lond 1951;A243:251±8. [33] Mooney M. J Appl Phys 1940;11:582. [34] Mark JE. Rubber Chem Technol 1975;48:495±508. [35] Ellis B, Welding GN. Techniques of polymer sciences. London: Soc Chem Ind, 1964. p. 46.
Role of dichlorocarbene modi®ed styrene butadiene rubber in compatibilisation of styrene butadiene rubber and chloroprene rubber blends M.T. Ramesan a, George Mathew b, Baby Kuriakose a, Rosamma Alex a,* a
Rubber Research Institute of India, Kottayam-9, Kerala, India b CMS College, Kottayam-1, Kerala, India Received 28 January 2000; accepted 13 June 2000
Abstract A study was conducted on the use of dichlorocarbene modi®ed styrene butadiene rubber (DCSBR) for the compatibilisation of blends of styrene butadiene rubber (SBR) and chloroprene rubber (CR). Stress±strain behaviour, mechanical properties, and low temperature transitions of the blends were examined in order to elucidate the eciency of the compatibiliser. It was found that eective compatibilisation was achieved when chlorine content of DCSBR was 25% and its dosage was 5 phr (parts per hundred rubber). Thermal analysis by DSC showed that an appreciable extent of molecular level miscibility has been achieved in SBR/CR blends by using DCSBR as a compatibiliser. The formation of interfacial crosslinks in the presence of DCSBR is evident from the cure characteristics and stress±strain isotherms. An increase in tensile strength, tear strength, resilience and hardness and a decrease in compression set was observed when the chlorine content and dosage of the compatibiliser increased from 15% to 25% and 5 phr, respectively. Compatibilised blends showed enhanced mechanical properties in the presence of reinforcing ®llers such as HAF carbon black and precipitated silica. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Compatibiliser; Dichlorocarbene modi®ed styrene butadiene rubber; Styrene butadiene rubber/chloroprene rubber blend
1. Introduction Blending of elastomers is being practised to achieve the best balance of physical properties, processability and cost. Elastomeric blends can either be miscible or immiscible depending on the free energy of mixing, but the number of miscible elastomer blends has been very few [1]. Blending of two incompatible polymers yields a material with poor mechanical properties. However, dierent studies have demonstrated that the mechanical properties of such blends can be signi®cantly improved by adding a suitable compatibiliser [2±6] during processing. These materials decrease the macroscopic in-
*
Corresponding author. Fax: +91-481-578-317.
homogeneities and increase the stability of the morphology during processing by reducing the interfacial tension and subsequently the dispersed phase size [7±10], as expected from Taylor's theory [11]. Speci®c interaction of polymer with functional groups can increase the extent of thermodynamic compatibility. Compatibility of polymer blends can also be improved by the use of copolymers containing segments chemically identical to the individual homopolymers [12±15]. If one of the polymers of a graft copolymer is miscible with one of the phases, then also the blend can be compatibilised [16]. Thermodynamic theories concerning the emulsifying eect of block copolymers in polymer blends have been developed by Leibler [17,18], and Noolandi and Hong [19±21]. Blends of SBR and BR were found to be more compatible after vulcanisation by Livingston et al. [22].
0014-3057/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 1 5 7 - 9
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Blends based on functionally active rubbers are reported to be miscible due to crosslinking reaction by the functional groups [23±25]. Chlorinated polyethylene acts as a compatibiliser in chloroprene rubber (CR)/ethylene propylene diene monomer (EPDM) rubber and acrylonitrile butadiene (NBR)/EPDM blends [26]. Polymethyl methacrylate (PMMA) acts as a compatibiliser in blends of NR/NBR [27]. CR is also a compatibiliser in NR/ NBR blends [27]. Blending of SBR with CR has been done to obtain better crystallisation resistance, better compression set resistance, lower brittleness temperature and enhanced resistance to sunlight deterioration as compared with CR alone. Other important properties, such as oil, heat, ¯ame and ozone resistance, decrease as the amount of SBR increases. It is possible to improve the phase morphology of SBR/CR blend by incorporating 5±10 phr of a modi®ed copolymer that has segments chemically identical to SBR and CR phases. Addition of halocarbenes to polymers has been reported since a long time; however, available information on the commercial application on such modi®ed polymers is limited. In this paper, eciency of dichlorocarbene modi®ed SBR as a compatibiliser in SBR/CR blends is reported. The eects of the level of modi®cations and the amount of modi®ed SBR required for compatibilisation are studied in detail. It is considered that there would be sucient interaction between the polar segments of modi®ed SBR with CR phase and butadiene part with SBR phase, thereby reducing the interfacial tension. 2. Experimental 2.1. Materials Styrene butadiene rubber (Synaprene 1502) having 25% bound styrene was obtained from Synthetics and Chemicals Ltd., Bareilly, UP, India. CR (Neoprene W type) having 36% chlorine content was obtained from Du Pont Co., USA. Dichlorocarbene modi®ed SBR (DCSBR) used as the compatibiliser was prepared by the reaction of dichlorocarbene produced by the alkaline hydrolysis of chloroform with SBR using phase transfer catalyst as reported earlier [28]. The level of modi®cation was monitored by the determination of chlorine content using chemical analysis techniques. The structural changes taking place during chemical modi®cation is shown in Scheme 1. Master batches of SBR and CR were prepared separately and then blended on a laboratory size two roll mixing mill (15 30 cm2 ) at a friction ratio 1:1.25 as per ASTM D-15-627. The temperature of the mill was maintained at 30°C by the circulation of cold water through the rolls. The compatibiliser of varying chlorine contents from 15% to 35% was added to preblended
Scheme 1. Structural changes of SBR during dichlorocarbene modi®cation.
SBR/CR (70/30, 50/50 and 30/70) at a constant dosage of 5 phr. The compatibiliser having 25% chlorine content was added at various dosages of 1±10 phr to 50/50 SBR/CR preblend compositions. Filled compounds were prepared by adding HAF (high abrasion furnace) carbon black and precipitated silica to the 50/50 blend compatibilised by 5 phr DCSBR containing 25% chlorine. The basic formulations used in the study are presented in Tables 1±3. Monsanto rheometric studies of the mixes (Fig. 1) were carried out at 150°C with 3° arc of rotor oscillation according to ASTM D-2705. The samples were vulcanised to their respective optimum cure time in a hydraulic press at 150°C and a pressure of 45 kg cmÿ2 on the mould. The tensile strength and tear resistance of the samples were tested with a Zwick universal testing machine (model-1474) at a temperature of 25 2°C and at a crosshead speed of 500 mm minÿ1 , according to ASTM D-412-80 and ASTM D-624-81, respectively. All other Table 1 Formulation of SBR/CR blends in the presence and absence of DCSBR Chemical
S7
S5
S3
S7
S5
S3
SBR CR DCSBR Zinc oxide Magnesium oxide Stearic acid CBSa TMTDb NA22c TDQd Sulphur
70 30 ± 5 1.2
50 50 ± 5 5
30 70 ± 5 2.8
70 30 5 5 1.2
50 50 5 5 2
30 70 5 5 2.8
1.15 0.7 0.15 0.15 0.7 1.54
1.0 0.5 0.25 0.25 0.5 1.1
0.8 0.3 0.35 0.35 0.3 0.66
1.15 0.7 0.15 0.15 0.7 1.54
1.0 0.5 0.25 0.25 0.5 1.1
0.8 0.3 0.35 0.35 0.3 0.66
*
DCSBR containing (a) 15%, (b) 20%, (c) 25%, (d) 30% and (e) 35% of chlorine content, respectively, were added to these blends. a Cyclohexyl benzathiazole. b Tetramethyl thiuram disulphide. c Ethylene thiourea. d 2,2,4-Trimethyl 1,2-dihydroquinoline.
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Table 2 Formulation of 50/50 SBR/CR carbon black ®lled blends in the presence and absence of DCSBR Chemical
CU1
CC1
CU2
CC2
CU3
CC3
CU4
CC4
SBR CR DCSBRa Zinc oxide Magnesium oxide Stearic acid CBS TMTD NA22 TDQ Aromatic oil HAF C-black Sulphur
50 50 ± 5 2 1 5 0.25 0.25 1.0 0.7 10 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 0.7 10 1.1
50 50 ± 5 2 1 5 0.25 0.25 1.0 1.4 20 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 1.4 20 1.1
50 50 ± 5 2 1 5 0.25 0.25 1.0 2.1 30 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 2.1 30 1.1
50 50 ± 5 2 1 5 0.25 0.25 1.0 2.8 40 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 2.8 40 1.1
a
DCSBR containing 25% of chlorine content.
Table 3 Formulation of 50/50 SBR/CR precipitated silica ®lled blends in the presence and absence of DCSBR Chemical
SU1
SC1
SU2
SC2
SU3
SC3
SU4
SC4
SBR CR DCSBRa Zinc oxide Magnesium oxide Stearic acid CBS TMTD NA22 TDQ Aromatic oil Precipitated silica Diethylene glycol Sulphur
50 50 ± 5 2 1 5 0.25 0.25 1.0 0.7 10 0.5 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 0.7 10 0.5 1.1
50 50 ± 5 2 1 5 0.25 0.25 1.0 1.4 20 0.5 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 1.4 20 0.5 1.1
50 50 ± 5 2 1 5 0.25 0.25 1.0 2.1 30 1 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 2.1 30 1 1.1
50 50 ± 5 2 1 5 0.25 0.25 1.0 2.8 40 1.5 1.1
50 50 5 5 2 1 5 0.25 0.25 1.0 2.8 40 1.5 1.1
a
DCSBR containing 25% of chlorine content.
technical properties were determined as per the relevant ASTM standards. The eciency of the compatibiliser was assessed by determination of Tg using a Perkin±Elmer dierential scanning calorimeter, operated at a heating rate of 15°C minÿ1 within the temperature range of ÿ80°C to 25°C.
3. Results and discussion
ÿ33°C. For the uncompatibilised blend, there appears to be two transitions in the temperature range from ÿ60°C to ÿ30°C which indicates the presence of microlevel inhomogeneity. The transitions at ÿ56°C and ÿ33°C for the blends correspond to the transitions of pure SBR and pure CR, respectively. However, the blend with 5 phr containing 25% chlorine and DCSBR shows a single transition in the temperature range ÿ55°C to ÿ33°C with a midpoint around ÿ40°C. This shows that DCSBR acts as a compatibiliser in blends of SBR/CR.
3.1. Thermal analysis
3.2. Processing characteristics from rheometric data
The DSC traces of pure SBR and CR and their 50/50 blend in the presence and absence of compatibiliser are shown in Fig. 1. Pure SBR shows a glass transition temperature (Tg ) at ÿ56°C and pure CR shows a Tg at
The maximum torque, optimum cure time, and scorch time change signi®cantly with blend composition as given in Table 4 and shown by the rheographs of the mixes in Fig. 2a±d. In all blend compositions, a
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crosslink density and chain entanglement. As a result of the comparatively high molecular weight of SBR, the long chain tends to coil. The dierent portions of the chain entangle with chain of CR and its own chains. High extent of entanglement causes an increase in the maximum torque value and hence the Mh-Mn value. The higher Mh-Mn value of the 70/30 SBR/CR blend compared to the 50/50 blend can also be due to a higher crosslink density achieved in 70/30 SBR/CR blend through better distribution of the curatives. There is a progressive increase in scorch time as SBR concentration in the blend increases. Crosslinking of CR in the presence of ZnO and accelerators such as TMTD and ETU proceed very rapidly, and ZnO or ZnCl2 formed catalyses the reaction [29]. The mechanism of crosslinking by sulphenamide accelerator in SBR is accompanied by a great processing safety. With addition of 5 phr DCSBR of chlorine content varying from 15% to 35%, the maximum torque attained increases with a decrease in scorch time in all blend compositions showing that there is a greater crosslinking. The presence of DCSBR brings about sucient adhesion between the phases [30] due to the polar interaction among blend constituents. Thus, there is a reduction of surface energy mismatch between polymers, and there is sucient interfacial crosslinks. This interfacial crosslink formation increases as the concentration of chlorine in DCSBR increases.
Fig. 1. DSC thermograms of 50/50 SBR/CR blend: (a) without compatibiliser and (b) compatibiliser with 5 phr loading.
comparatively high optimum torque is obtained when the concentration of CR in the blend is high. It is already reported that the maximum torque is dependent on Table 4 Cure characteristics of SBR/CR blends compatibilised by DCSBR Blend composition
Chlorine content of DCSBR (%)
Optimum cure time at 150°C (t90 ) (min)
SBR/CR 70/30
0 15 20 25 30 35
26.5 28 29 30 26 25
5.5 5 4.75 4.5 4.25 4
30 32 33 34 34 33
SBR/CR 50/50
0 15 20 25 30 35
22 24 25.5 27 25 24
4 3.75 3.5 3.25 3 2.8
28 29 30 32 31 30
Dosage of compatibiliser (phr) having 25% chlorine content in 50/50 blend
1 3 5 10
23 25 27 25
3.75 3.5 3.25 3
29 30 32 29
SBR/CR 30/70
0 15 20 25 30 35
16 18.5 20 21 18 16
3 2.75 2.5 2 1.75 1.5
31 33 34 36 35 35
Scorch time at 150°C (t2 ) (min)
Mh-Mn (dN m)
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723
Fig. 2. Rheographs of (a) 70/30 SBR/CR blend and compatibilised blend containing 5 phr DCSBR of varying chlorine contents at 150°C: a. 15%, b. 20%, c. 25%, d. 30% and e. 35%; (b) 50/50 SBR/CR blend and compatibilised blend containing 5 phr DCSBR of varying chlorine content at 150°C: a. 15%, b. 20%, c. 25%, d. 30% and e. 35%; (c) 30/70 SBR/CR blend and compatibilised blend containing 5 phr DCSBR of varying chlorine content at 150°C: a. 15%, b. 20%, c. 25%, d. 30% and e. 35%; (d) 50/50 SBR/CR blend and compatibilised blend containing dierent dosage of DCSBR (25%) at 150°C: a. 0 phr, b. 1 phr, c. 3 phr, d. 5 phr and e. 10 phr.
As the concentration of SBR in the blend increases, the cure characteristics such as optimum cure time, scorch time and induction time at 150°C increases for the uncompatibilised blends. The mechanism of vulcanisation is dierent in the two rubbers, and polychloroprene compounds are known to be scorchy. Since the amount of accelerator is ®xed in these compositions, the curing properties suer an increase. The higher scorch time of 70/30 SBR/CR blend reveals its comparatively better scorch safety. There is an increase in optimum cure time as chlorine content of DCSBR increases up to 25%, and beyond that level of chlorination, the optimum cure time decreases. When the chlorine content is 25% and the loading is 5 phr, the compatibilising eciency is maximum (as evidenced from tensile property). This and compatibilisation can be due to microlevel interactions large interfacial area formed by the uniformly distributed microdomains. Since more reactants are needed for
the formation of a greater number of interface crosslinks, the optimum cure time is found to increase. But as the chlorine content increases further, the compatibiliser eciency seems to decrease, which ultimately ends or results in the coalescence of dispersed domains. This reduces the interfacial area and thereby causes a decrease in optimum cure time. The eect of dosage of compatibiliser on rheometric torque is shown in Fig. 2d. The maximum torque attained increases with the loading of the compatibiliser. However, at a dosage of 10 phr, the maximum torque is only marginally higher than that obtained with 5 phr DCSBR. It is clear that as the loading of the compatibiliser increases, there is greater interaction among the constituents promoting interfacial crosslinks. The optimum cure time increases as the dosage of DCSBR increases up to 5 phr, and beyond this dosage, the value drops. Hence, optimum vulcanisation characteristics are obtained with DCSBR of chlorine content 25% and at a dosage of 5 phr.
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3.3. Calculation of crosslink density 3.3.1. Stress±strain isotherms The extent of physical crosslinks in an elastomer vulcanisate can be assessed by using Mooney Rivlin equation [31±33]. The equation is given by F 2A0 k ÿ kÿ1 C1 kÿ1 C2 ;
1
where F is the extension force required to stretch a piece of rubber vulcanisate (dumb-bell) of cross-section area, A0 to an extension ratio, k. A plot of F =2A0 k ÿ kÿ1 vs. kÿ1 gives a straight line whose y intercept C1 is directly related to the physically eective crosslink density, mphys , by the equation C1 qRT mphys :
2
The plots obtained are presented in Fig. 3, and the values of crosslink density are shown in Table 5. It can be seen that the force and crosslink density increases as the concentration of DCSBR increases in 50/50 blend and is in agreement with the increase in rheometric torque. C2 , a term which serves as a measure of departure of observed stress±strain relationship from the form suggested by statistical theories [34], is given in Table 5. 2C2 value increases with the loading of compatibiliser up to 5 phr and then decreases. The observation of a higher 2C2 for the 50/50 blend containing 5 phr DCSBR shows the presence of higher chain entanglement [34]. Since the elastomer matrix is composed of two components, a higher entanglement shows better molecular level mixing. At higher concentrations of DCSBR, there is probably less chain entanglement and hence molecular level mixing though there is a much enhanced polar interaction. Here, it acts as a third component. 3.3.2. Swelling studies Equilibrium swelling in toluene to assess the volume fraction Vr of the rubber network in the swollen gel was calculated by the method reported by Ellis and Welding [35] and is presented in Table 5.
Fig. 3. Plots of F =2A0 k ÿ kÿ1 vs. kÿ1 of 50/50 SBR/CR blend with dierent dosages of compatibiliser.
Vr
d ÿ fwqÿ1 r ; ÿ1 d ÿ fw qÿ1 r A0 qs
3
where d is the weight after drying out the sample, and w is the weight of swollen sample. A0 is the weight of the absorbed solvent, f, the fraction of insoluble components, qr and qs , the densities of the rubber and solvent, respectively. Vr increases with the addition of compatibiliser progressively up to 5 phr and at still higher concentrations, the value of Vr drops. This is due to the decrease in the extent of swelling caused by higher polymer±polymer interactions and increased interfacial crosslinks formed in the presence of compatibiliser. This is in good agreement with the crosslink density obtained from the stress±strain data. 3.4. Eect of compatibiliser on technological properties The physical properties of the blends are given in Tables 6±8. In pure blends, tensile strength, elongation at break and modulus increase as CR concentration
Table 5 Crosslink density parameters C1 , C2 , mphys , Vr and dierence in rheometric torque of 50/50 SBR/CR blend with varying concentration of compatibiliser Compatibiliser loading (phr)
mphys 10ÿ3 (g mol mlÿ1 )
Mh-Mn (dN m)
2C2 (N mmÿ2 )
2C1 (N mmÿ2 )
Vr (from swelling studies)
0 1 3 5 10
1.06 1.22 1.98 2.75 2.09
28 29 30 32 29
1.04 2.32 3.36 4.0 3.12
0.536 0.686 0.750 0.890 0.82
0.3364 0.3568 0.3685 0.3874 0.3766
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725
Table 6 Mechanical properties of blends of 70/30 SBR/CR compatibilised by DCSBR of chlorine content varying from 15% to 35% Properties
Percentage of chlorine in DCSBR used at 5 phr loading (1)
(2)
(3)
(4)
(5)
(6)
Modulus, 300% (MPa) Tensile strength (MPa) Elongation at break (%) Tear strength (kN mÿ1 ) Hardness (Shore A) Resilience (%) (Dunlop tripsometer) Compression set (%) 22 h at 70°C
1.7 3.2 430 19.4 31 62
1.8 3.3 370 16.6 33 63
1.82 3.4 380 18.8 35 64
1.67 3.8 500 19.1 36 65
1.83 3.5 420 19.8 38 64
1.85 3.19 390 19.4 39 63
18.6
17.5
16.6
15.4
15.8
16.9
In (2)±(6), the DCSBR blends contain 15%, 20%, 25%, 30% and 35% of chlorine content, respectively.
Table 7 Mechanical properties of blends of 50/50 SBR/CR compatibilised by DCSBR of chlorine content varying from 15% to 35% Properties
Percentage of chlorine in DCSBR used at 5 phr loading (1)
(2)
(3)
(4)
(5)
(6)
Modulus, 300% (MPa) Tensile strength (MPa) Elongation at break (%) Tear strength (kN mÿ1 ) Hardness (Shore A) Resilience (%) (Dunlop tripsometer) Compression set (%) 22 h at 70°C
1.6 3.9 520 16.7 33 60
1.7 4.6 550 16.0 33 61
2.1 5.73 580 19.1 36 63
2.94 6.5 680 20.9 39 64
2.48 5.8 600 20.4 41 62
2.0 5.2 490 19.0 43 61
20.8
20.8
19.4
18.1
20.2
22.0
In (2)±(6), the DCSBR blends contain 15%, 20%, 25%, 30% and 35% of chlorine content, respectively.
Table 8 Mechanical properties of blends of 30/70 SBR/CR compatibilised by DCSBR of chlorine content varying from 15% to 35% Properties
Percentage of chlorine in DCSBR used at 5 phr loading (1)
(2)
(3)
(4)
(5)
(6)
Modulus, 300% (MPa) Tensile strength (MPa) Elongation at break (%) Tear strength (kN mÿ1 ) Hardness (Shore A) Resilience (%) (Dunlop tripsometer) Compression set (%) 22 h at 70°C
2.39 8.10 690 25 28 53 32.5
1.71 6.9 650 20.5 31 54 31.6
1.31 7.0 750 21.6 33 54 28.8
1.80 8.4 800 25.8 35 55 25.8
1.28 6.4 750 21.7 37 52 27.8
1.65 6.6 700 18.9 38 51 28
In (2)±(6), the DCSBR blends contain 15%, 20%, 25%, 30% and 35% of chlorine content, respectively.
increases. The resilience shows a decrease, whereas compression set increases with the increase in CR concentration in the blends. Mechanical properties in all blend compositions improve with addition of 5 phr DCSBR of chlorine content varying from 15% to 35%. The level of improvement depends on the chlorine content of DCSBR. Tensile strength and elongation at
break, hardness and resilience increase while compression set decreases with an increase in chlorine content of DCSBR up to 25% and then decreases. Ecient compatibilisation by 25% chlorine containing DCSBR creates strong interfacial crosslinks which result in an increase in tensile strength. The improvement in mechanical properties is more eective at 50/50 blend
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Table 9 Mechanical properties of blends of 50/50 SBR/CR compatibilised by DCSBR Properties
Loading of 25% chlorine containing DCSBR (phr) 0
1
3
5
10
Modulus, 300% (MPa) Tensile strength (MPa) Elongation at break (%) Tear strength (kN mÿ1 ) Hardness (Shore A) Resilience (%) (Dunlop tripsometer) Compression set (%) 22 h at 70°C
1.66 3.9 520 16.7 33 60 20.8
1.98 4.6 400 17 34 62 19.3
2.35 5.73 440 18.4 37 63 18.8
2.94 6.5 680 20.9 39 64 18.1
2.8 5.8 570 21.3 42 61 21.3
composition. A comparatively higher tensile strength, elongation at break and tear strength is obtained for the compatibilised 30/70 SBR/CR blend. The microscopic extension ratio of the dispersed CR domains is much higher than the macroscopic extension ratio during tensile test. Greater extension ratios cause strain induced crystallisation in CR which results better tensile properties in these cases. Owing to the absence of ecient interfacial crosslink, the uncompatibilised blend shows comparatively poor mechanical properties. The tensile strength values for compatibilised SBR/CR 70/ 30 are found to comparatively lower due to higher proportions of amorphous and weak SBR in this blend . The tensile strength of 50/50 composition increases with the loading of compatibiliser (Table 9) up to 5 phr, and with further increase in the concentration of compatibiliser, tensile strength does not change much. This may be due to the supersaturation of the interphase with the compatibiliser which increases interfacial tension.
Fig. 4. Rheographs at 150°C of carbon black ®lled 50/50 SBR/ CR blend and compatibilised blend.
3.5. Eect of dierent ®llers on compatibilised SBR/CR (50/50) blends The rheographs of carbon black and silica ®lled blends in the presence and absence of compatibiliser are presented in Figs. 4 and 5. The rheometric torque increases with the addition of 5 phr of DCSBR. The optimum cure time increases, whereas scorch time decreases with the addition of ®ller. The carbon black ®lled blends shows a comparatively higher Mh-Mn value, higher optimum cure time and lower scorch time as compared to silica ®lled blends. There is enhancement in technical properties with the addition of ®ller as shown in Tables 10 and 11. The blend containing ®llers of carbon black and silica shows a higher tensile strength, tear strength and abrasion resistance as compared to uncompatibilised blends. This is due to the improvement in polymer ®ller interaction resulting from the eective interfacial adhesion and the uniform distribution of the phases.
Fig. 5. Rheographs at 150°C of silica ®lled 50/50 SBR/CR blend and compatibilised blend.
The heat build-up and compression set values decreases whereas resilience increases on compatibilisation. The compatibiliser present in the blend reduces the heat build-up due to the formation of eective interface which avoids severe friction between component and polymer ®ller friction. The heat build-up increases with the increase in ®ller loading, and the eect is maximum for the carbon black ®lled samples.
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Table 10 Mechanical properties of compatibilised 50/50 SBR/CR blend with dierent loading of carbon black ®ller Properties
CU1
CC1
CU2
CC2
CU3
CC3
CU4
CC4
Modulus, 300% (MPa) Tensil strength (MPa) Elongation at break (%) Tear strength (kN mÿ1 ) Din abrasion loss (mm3 ) Heat build-up (°C) Compression set (%) 22 h at 70°C Resilience (%) (Dunlop tripsometer)
3.2 8.0 560 31.3 196 31 16.8 60
4.1 9.5 540 41.0 162 28 15.2 60
3.9 11.8 530 46.0 129 37 15.4 52
4.8 14.8 510 50.6 105 33 13.0 58
5.3 16.5 500 55.1 111 42 13.4 49
6.1 19.0 475 57.1 93 36 11.2 55
5.8 19.8 560 56.2 96 47 12.4 41
6.7 21.2 430 62.0 70 43 10.9 50
Table 11 Mechanical properties of compatibilised 50/50 SBR/CR blend with dierent loading of silica ®ller Properties
SU1
SC1
SU2
SC2
SU3
SC3
SU4
SC4
Modulus, 300% (MPa) Tensil strength (MPa) Elongation at break (%) Tear strength (kN mÿ1 ) Din abrasion loss (mm3 ) Heat build-up (°C) Compression set (%) 22 h at 70°C Resilience (%) (Dunlop tripsometer)
2.5 5.8 580 23.7 254 29 24 64
3.0 7.8 570 25.9 216 25 21 65
2.9 10.1 550 25.5 181 32 21 62
3.7 12.3 530 28.4 150 28 19 64
3.5 11.2 540 30.6 132 36 20 57
4.1 13.0 490 32.8 119 32 18 62
4.0 14.1 470 35.4 114 38 18 52
4.9 15.0 455 37.8 109 34 16 60
4. Conclusion DCSBR acts as a compatibiliser in the otherwise incompatible blends of SBR/CR. The compatibilisation is accompanied by an improvement in mechanical properties and depends on chlorine content and concentration of DCSBR. The compatibilising action is more ecient in 50/50 blend and increases progressively with chlorine content of compatibiliser up to 25% and a dosage up to 5 phr, beyond which there is no further increment. The presence of chain entanglements as revealed from stress±strain isotherm is due to better molecular level mixing and leads to enhanced physical properties. The incorporation of ®ller imparts enhanced the physical properties in compatibilised SBR/CR blends. References [1] Roland CM. In: Bhowmick AK, Stephens HL, editors. Handbook of elastomers new developments technology. New York: Marcel Dekker, 1988. [2] Hess WM, Scott CE, Callan JE. Rubber Chem Technol 1967;40:371±82. [3] Marsh PA, Voet A, Price LD. Rubber Chem Technol 1967;40:359±63. [4] Fayt R, Jerome R, Teyssie PH. J Polym Sci Part B Polym Phys 1982;20:2209±17. [5] Joseph A, George KE. Kunststoe 1991;44:538±41.
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