polybutadiene blends

April 2002

Materials Letters 53 (2002) 268 – 276 www.elsevier.com/locate/matlet

Melt elasticity and extrudate characteristics of polystyrene/polybutadiene blends Susan Joseph a, Zachariah Oommen b, Sabu Thomas c,* a

St. Stephen’s College, Pathanapuram, Kerala, 689695, India b C. M. S. College, Kottayam, Kerala, India c School of Chemical Sciences, Priyadarshini Hills P.O., Mahatma Gandhi University, Kottayam, Kerala, 686560, India Received 20 May 2001; accepted 26 June 2001

Abstract The melt elasticity characteristics of polystyrene/polybutadiene (PS/PB) blends have been studied with reference to the effect of blend ratio, shear rate, and temperature. The effect of compatibilization and dynamic vulcanization on melt elasticity behavior of the blends has also been studied. It is found that compatibilization and dynamic vulcanization considerably affect melt elastic properties. Morphological analysis by scanning electron microscopy (SEM) also provides complimentary evidences. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Polystyrene; Polybutadiene; Melt elasticity; Compatibilization; Dynamic vulcanization

1. Introduction Blending two or more kinds of polymers is one way of developing materials with properties superior to those of individual constituents. It is important especially from an industrial point of view to control the state of mixing and the phase-separated structure of polymer blends. Blends of rubber and plastic known as thermoplastic elastomers (TPEs) combine the excellent processability characteristics of thermoplastics and the mechanical properties of vulcanized rubbers. The improvement in properties by blending is possible by lowering the stock viscosity or producing materials that is less prone to fracture, when *

Corresponding author. Tel: +91-481-558303 (office); fax: +91-481-561190. E-mail address: [email protected] (S. Thomas).

subjected to flow. It is also possible to alter the normal stress functions and related phenomenon by suitable blending technique. Systematic studies of melt elasticity are necessary for producing superfine finished goods. For evaluating the processability, a thorough understanding of the melt elasticity, melt fracture, extrudate deformation, etc. has to be taken into account. Thermoplastic elastomers from blends of polyCstyrene (PS) and polybutadiene (PB) are of much importance. PS exhibits superior processing characteristics and has high modulus and good dielectric properties. However, it has the limitation of being highly brittle. Good elastic properties, good resilience, good weather resistance, resistance to abrasion and good damping behavior but poor chemical resistance and processability characterize PB. The properties of PS can be improved by blending with

0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 1 ) 0 0 4 9 1 - 8

S. Joseph et al. / Materials Letters 53 (2002) 268–276

PB. Thus, we can alleviate many of the limitations encountered with both the homopolymers and can produce super tough, high-performance engineering thermoplastic elastomers, which combine the excellent processing characteristics of PS and the elastic properties of PB. Blends of PS and PB are a new class of TPEs that exhibit better processability, impact strength, and good flexibility. Both PB and PS possess different melt elasticity and therefore in order to optimize the processing conditions for these blends, it is necessary to study the effect of shear stress at different temperatures on melt elasticity. The effect of compatibilization and dynamic vulcanization on melt elasticity behavior of the blends has also been taken into account. The unmodified PS/PB blend shows typical behavior for immiscible blends. Most of the polymer pairs are found to be immiscible and incompatible. The problem of poor interfacial adhesion and macro phase separation can be alleviated mainly by two approaches in our present work, viz. (i) the physical approach by incorporation of a suitable compatibilizer and (ii) by chemical approach of dynamic vulcanization. Compatibilizers are potential interfacial modifiers, acting on the interface of immiscible polymer blends. Both block and graft copolymers act as potential compatibilizers, and when added to an immiscible blend, act as interfacial agents. This technique promotes dispersive mixing and stabilizes the morphology. The addition of a suitably selected compatibilizer to an immiscible binary blend reduces the interfacial energy between the phases, permits finer dispersion during melt mixing, provides stability against phase segregation, and results in improved interfacial adhesion. Enhanced performance of blends can be achieved by reducing the immiscibility between the polymer components, maintaining a twophase morphology with improved interfacial adhesion [1]. Compatibility is a desirable property in the industrial field for use in fabricability and resistance to gross phase segregation during the cooling down process of molten mixture. It is best known that the elastomer particles should be cross-linked to promote elasticity. Dynamic crosslinking is another important reactive polymer processing method, which can be applied to thermoplastic elastomers [2– 4] to promote elasticity. In this process, the small elastomer droplets are vulcanized to give a

269

particulate vulcanized elastomer phase of stable domain morphology during melt – mixing process, and subsequent dynamic vulcanization provides compositions that have improved properties. When the elastomer phase is preferentially cross-linked using different cross-linking agents, they acquire distinct characteristics depending on the type of cross-link generated. The main features of these TPEs are the ability to be melt-processed as cross-linked thermoplastics and outstanding elastic recovery after much deformation. Dynamic vulcanization leads to reduced permanent set, improved ultimate mechanical properties, greater fluid resistance, improved high temperature utility, greater stabilization of phase morphology, greater melt strength, more reliable thermoplastic fabricability and improved fatigue resistance [5 –7]. Both these techniques provide a means for costeffective production of new multiphase polymeric systems. Several authors have reported the morphology and viscosity – compatibility relationship of these blends. This includes the work of Chu et al. [8] and Spiegelberg et al. [9]. Polystyrene (PS)-based blends have received increasing commercial attention with regard to their physical and mechanical properties through compatibilization of the polymer components of the blends [10,11]. To our knowledge, to this date, no detailed study has been made on the melt elasticity properties of the uncompatibilized, compatibilized and dynamically vulcanized TPEs made from polystyrene and polybutadiene. The main objective of this article is to study the melt elasticity characteristics of PS/PB blends as a function of blend ratio, compatibilizer loading and dynamic vulcanization over a wide range of shear stresses and temperatures. The effects of blend ratio, shear rate and temperature on the melt elasticity of the system have been studied in detail. The effects of compatibilization by styrene butadiene rubber (SBR) and dynamic vulcanization using different cross-linking systems on melt elasticity parameters is also studied in detail. It is well known that block copolymers of appropriate architectures can be utilized as compatibilizers for modifying the interfacial properties of this binary blend system. Styrene butadiene rubber (SBR) is a random block co-polymer which is non-reactive. It has segments identical with each of the blend components.

270

S. Joseph et al. / Materials Letters 53 (2002) 268–276

2. Experimental

2.3. Rheological measurements

2.1. Materials used

Melt rheology of the blend components and blends was characterized by measuring their viscosities in shear flow using a capillary rheometer, Gottfert (2002), with a 30 l/d ratio and a 180B angle of entry. The measurements were made at 180 BC. For S70 and S100, studies were also made at 190 and 200 BC. The shear rates were varied from 5 to 300 s
1.

Polystyrene (PS) was supplied by Poly Chem., Bombay, India and cis-1,4 polybutadiene was obtained from IPCL Vadodara. Styrene butadiene rubber (SBR) is a random co-polymer of styrene and butadiene (0.97 g/cm3 density with 23% styrene content) that was used as the compatibilizer. All other ingredients such as dicumyl peroxide, tetra methylthiuram disulfide, etc. are of laboratory reagent grade.

2.4. Die swell measurements The small pieces of the extrudate from the capillary are carefully collected, avoiding any stretching. For calculating die swell values, the diameter of the extrudate was determined using a travelling microscope.

2.2. Preparation of blends Blends of PS and PB were prepared in a Haake Rheocord 90 mixer. The temperature, rotor speed and mixing time were 180 BC, 60 rpm and 6 min, respectively. The melt mixed samples are denoted as S00, S20, S30, S40, S50, S60, S70, S80 and S100, where S stands for melt-mixed system and the subscripts indicate polystyrene content in the blend. The compatibilized blends of S70 with 1, 2.5, 5, 10 and 20 wt.% of SBR with respect to minor phase PB, are denoted as SC7001, SC702.5, SC7005, SC7010 and SC7020, respectively. Dynamically vulcanized blends were prepared under identical conditions as those of corresponding unvulcanized compositions, with a difference that after the initial mixing for 5 min, other ingredients according to the formulations given in Table 1 are added. They are denoted as S70C, S70E, S70M, and S70P for conventional, efficient, mixed and peroxide systems, respectively.

Important parameters that characterize the melt elasticity of polymer blends are extrudate swell, principal normal stress difference (s11
s22) and recoverable shear strain (SR). Among the different parameters that determine the properties of an extruded strand, melt elasticity characteristics are more important than melt viscosity aspects. Die swell (de/dc) is associated with elastic memory. It is the ratio of the extrudate diameter (de) to the diameter of the capillary (dc). The principal normal stress difference is calculated from the die swell values and shear stress according to Tanner [12],

Table 1 Formulations of dynamic vulcanized blends

SR ¼ ðs11
s22 Þ=2s

Components

S70C

S70E

S70M

S70P

PS PB ZnO Stearic acid MORa S DCPb TMTDc

70 30 1.5 0.6 0.6 0.6 nil nil

70 30 1.5 0.6 0.75 0.09 nil 0.45

70 30 1.5 0.6 0.75 0.75 0.075 nil

70 30 nil nil nil nil 0.5 nil

a b c

4-morpholinyl-2-hexyl benzo thiazyl disulphide. Dicumyl peroxide. Tetra methyl thiuram disulfide.

2.5. Melt elasticity

s11
s22 ¼ 2sw ½2ðde =dc Þ6
21=2

ð1Þ

Recoverable shear strain (SR) was calculated [13] from the following equation ð2Þ

2.6. Blend morphology 2.6.1. Extrudate morphological observations The extruded samples collected in the rheological tests were used for morphological observations. The test samples were cryo-fractured in liquid nitrogen in order to have a brittle fracture, thus avoiding large deformations in the surface to be examined by a scanning electron microscope (SEM). For PS-rich blend, the dispersed PB phase was etched with n-

S. Joseph et al. / Materials Letters 53 (2002) 268–276

271

heptane, to obtain better insight into the blend morphology. The fracture surface was dried, coated with a thin layer of gold. After gold coating, the morphology was examined with a Philips XL20 model scanning electron microscope operating at an acceleration voltage of 20 kV. Micrographs of different magnifications and several fields of view were taken. The surface characteristics of the extrudates were studied by optical microscopy.

Table 2 Die swell values of S70 at different temperatures and shear rates Temperature (BC) Shear rates

2.6.2. Analysis of morphological parameters The SEM photographs were used to determine the sizes of dispersed rubber particles and PS particles and the particle size distribution. Various average domain diameters were obtained by measuring a large number of different domain diameters from different micrographs.

related to its elastic properties. The polymer chains get oriented due to shear, as the molten polymer flows through the capillary. As it emerges from the capillary, polymer molecules tend to recoil, and lateral expansion takes place, thus leading to the phenomenon of die swell or extrudate swell. This is a relaxation effect due to the recovery of the elastic deformation imposed in the capillary. Die swell characteristics of the uncompatibilized blends at three different shear rates are shown in Fig. 1. Die swell value of PB is not reported as it gives irregular extrudates since the elastic response increases with increase in rubber concentration. It is evident from the figure that the die swell value increases with the weight percentage of polystyrene. There is an increase in die swell with shear rate for all the blend compositions. This is associated with considerable increase in the recoverable elastic energy of the system at high shear rates [14]. There is also melt fracture, at high shear forces, where the shear rate exceeds the strength of the melt. Die swell also increases with increase in temperature, as shown in Table 2 for S70 blend. S20 and S30 blends show typical melt fracture at the lowest shear rates. This phenomenon is anticipated since PS tends to form a sheath in the extrudate due to lower viscosity of PS. The thin film gets fractured in the die head as the extrudate leaves the capillary at a slow extrusion rate and appears as flakes on the extrudate surface. Fig. 2 gives the die swell values of compatibilized blends. In compatibilized blends, die swell values are found to decrease considerably with compatibilizer loading. This is because the interface becomes stronger as a result of the addition of compatibilizer. It can also be explained on the basis of morphological changes with compatibilizer loading. In uncompatibilized blends, the rubber particles undergo a large extent of deformation in the die or are broken down into finer particles with increase of shear rates, so that

3. Results and discussion 3.1. Extrudate swell: effect of blend ratio, shear rate and temperature The extrudate swelling emerging from a capillary is a characteristic of non-Newtonian systems and is

Fig. 1. Deformation characteristics of the uncompatibilized blends at 180 BC and different shear rates.

180 190 200

50 s
1

100 s
1

200 s
1

1.32 1.37 1.38

1.37 1.38 1.46

1.45 1.47 1.50

272

S. Joseph et al. / Materials Letters 53 (2002) 268–276 Table 4 Melt elastic properties of uncompatibilized, compatibilized, and dynamically vulcanized blends extruded at a shear rates of 100 and 200 s
1 at 180 BC Blends

Shear rates 100 s
1

Fig. 2. Die swell values of compatibilized blends at 180 BC and different shear rates.

after extrusion they show high extent of recovery. However, better dimensional stability of the extrudate is possible by compatibilization. Similar observations of better dimensional stability are reported in natural rubber/polystyrene (NR/PS) [15] blends upon compatibilization with a graft co-polymer NR-g-PS. 3.2. Effect of shear rate and dynamic cross-linking on die swell ratio The swelling indexes of the dynamically crosslinked blends at 180 BC and at three different shear rates are shown in Table 3. In all the dynamically

Table 3 Die swell values of dynamically vulcanized S70 blends at 180 BC and at different shear rates Blends

S70C S70E S70M S70P

Shear rates 50 s
1

100 s
1

200 s
1

1.31 1.30 1.11 1.29

1.36 1.35 1.15 1.36

1.44 1.37 1.21 1.41

S100 S80 S70 S60 S50 S40 S30 S20 S7001 S702.5 S7005 S7010 S7020 S70C S70E S70M S70P

200 s
1

s11
s22 (  103 Nm
2)

SR

s11
s22 (  103 Nm
2)

SR

1144 1197 1235 1363 1556 1712 2187 2485 823 796 996 788 739 989 1107 527 882

5.58 4.15 3.74 3.58 3.50 3.43 3.35 3.28 3.18 3.09 2.93 2.85 2.77 3.26 3.18 1.62 3.26

1827 1778 1822 1960 2140 2300 2576 2913 1278 1223 1405 1081 977 1487 1380 835 1302

6.93 4.89 4.42 4.15 3.98 3.81 3.58 3.35 3.89 3.79 3.70 3.18 3.09 3.98 3.35 2.07 3.70

cured samples, die swell ratio shows a substantial reduction. According to Coran et al. [16], in dynamically cured TPEs, the die swell values are very low or even absent, since the viscosity can approach infinity at zero shear rates in the types of thermoplastic vulcanizate emerging from the die. The die swell occurs as a result of molecular relaxation upon emergence from the capillary. Vulcanization leads to network formation, thereby restricting the relaxation process. In a vulcanized sample, molecules cannot slip past each other as in unvulcanized materials. In dynamically vulcanized blends, there is reduction in extrudate deformation. As expected, die swell of dynamically vulcanized blend increases with increase

Table 5 Melt elasticity of S70 extruded at shear rates of 100 and 200 s
1 at 180 BC Temperature (BC)

s11
s22 (  103 Nm
2)

SR

190 200

1150 984

3.81 4.16

S. Joseph et al. / Materials Letters 53 (2002) 268–276

in shear rate. This is because, when the material is subjected to high pressure at high shear rates, the residence time of the material in the capillary decreases and, hence, the polymer stores more elastic energy. George et al. [17] have made similar observations of reduction in die swell upon dynamic vulcanization of PP/NBR blends. 3.3. Principal normal stress difference (s11
s22) The principal normal stress difference (s11
s22) and recoverable shear strain (SR) values of uncompa-

273

tibilized blends at 100 and 200 s
1 for different blend compositions are given in Table 4. The high (s11
s22) values indicate the high elasticity of the blends. As the rubber content increases, the blends exhibit high (s11
s22) values. As the shear rate increases, there is an increase in normal stress values also. Evidently, the high values of normal stress difference indicate greater elastic recovery or melt elasticity. The principal normal stress values of S70 at a shear rate of 100 s
1 and at 190 and 200 BC are shown in Table 5. The values decrease with increase of temperature. Compatibilized blends have lower

Fig. 3. Optical photographs of extrudates of (a) uncompatibilized, (b) compatibilized, and (c) dynamically vulcanized blends at 180 BC and at 100 s
1.

274

S. Joseph et al. / Materials Letters 53 (2002) 268–276

principal normal stress values than its uncompatibilized counterpart. The incorporation of a compatibilizer makes the blend less deformable, and hence, produces high melt viscosity and greater resistance to flow. Among the different curing systems, conventional and efficient vulcanized blends show higher principal normal stress values. There is an increase in normal stress values on increase in the shear rate.

energy surrounding the matrix, leading to smaller values of recoverable shear strain. In the case of compatibilized systems, SR decreases progressively with compatibilizer loading. Among the different curing systems, the conventional system shows a slightly higher value of SR. For all the systems studied, SR values increase with increase in shear rate. 3.5. Extrudate deformation studies

3.4. Recoverable shear strain (SR) Recoverable shear strain is a measure of elastic energy stored in the material. The excess energy may be converted into surface-free energy, which leads to extrudate deformation [18]. The value of recoverable shear strain progressively decreases with incorporation of PB and decreases with an increase of temperature. Polystyrene has a very high SR value, which decreases progressively on blending with PB and increases with an increase in temperature. This indicates poor recoverability of PB drops dispersed in PS matrix. These droplets may absorb a part of the strain

Fig. 3(a), (b), and (c) are optical photographs of extrudates of uncompatibilized, compatibilized and dynamically vulcanized blends at 180 BC and at different shear rates of extrusion. At low shear rates, most of the extrudates have smooth surfaces. However, at high shear rates, most of the extrudate surface exhibits a relatively different degree of distortion. At high shear rates, the surface is not as smooth as in the case of low shear rates. Thus, these extrudates have rough surfaces and non-uniform diameter. This is because of the melt fracture that occurs in the elastically deformed polymer, in which the shear stress

Fig. 4. Scanning electron micrographs of (a) uncompatibilized (b and c) compatibilized S70 blends at 180 BC and at 100 s
1.

S. Joseph et al. / Materials Letters 53 (2002) 268–276 Table 6 Average domain diameters of dispersed phase in uncompatibilized S70 and its compatibilized counterparts S7005 and S7020 PS/PB blends at 180 BC and 100 s
1 Blend

Dn (Am)

Dw (Am)

Da (Am)

Dv (Am)

Ds (Am)

PDI

S70 S7005 S7020

3.6 1.70 3.3

4.62 3.14 5.3

3.95 2.3 4.18

5.01 9.17 7.97

10.8 5.34 6.11

1.28 1.84 1.6

exceeds the strength of the melt [14]. The successive slipping and sticking of the polymer layer at the wall of the capillary is also a contributing factor for extrudate deformation [19]. The surface irregularity increases with increase in rubber content as seen for PB-rich blends. This is attributed to the decreasing melt strength and the increasing viscosity due to the soft rubbery nature of PB. It is evident from Fig. 3(b) and (c) that compatibilization reduces the die swell. This can be explained on the basis of morphological changes accompanied with compatibilizer loading. An attempt to reduce the extrudate distortion by reducing the shear stress or shear rate would adversely affect the flow rate, which in turn has its negative influence on production rate in polymer processing industry. Dynamic vulcanization using different curing agents also gives the same observation. Compatibilization and dynamic vulcanization are extremely useful for reducing the die swell. The effect of compatibilizer loading on the morphology of uncompatibilized and compatibilized counterparts S7005 and S7020 at a shear rate of 100 s
1 is shown in Fig. 4(a) to (c), respectively. There is a substantial decrease in the domain size of the dispersed-phase with compatibilizer loading (Table 6). Also, compatibilization makes the particle size more uniform. This indicates that SBR brings down the interfacial tension between PS/PB phases and aids in finer dispersion of PB in the polymer matrix, which also provides supportive evidences for the observed melt elastic properties.

4. Conclusion The swelling index values increase with the weight percentage of polystyrene. There is an increase in die

275

swell with shear rate for all the blend compositions, which is associated with considerable increase in the recoverable elastic energy of the system at high shear rates. For PB-rich blends, melt fracture is observed at high shear forces, where the shear rate exceeds the strength of the melt. Die swell also increases with an increase in temperature. The surface irregularity increases with increase in rubber content, which can be attributed to the decreasing melt strength and the increasing viscosity due to the soft rubbery nature of PB. Compatibilization and dynamic vulcanization reduce the extrudate deformation. Reduction in extrudate deformation upon compatibilization is better explained on the basis of morphological changes accompanied with compatibilizer loading. Nomenclature de/dc Die swell s11
s22 Principal normal stress difference SR Recoverable shear strain de Diameter of extrudate dc Diameter of the capillary s
1 Shear rate

References [1] J.A. Brydson, Flow Properties of Polymer Melts, 2nd edn., George Godwin, London, 1981, p. 963. [2] M. Saleem, W.E. Baker, J. Appl. Polym. Sci. 39 (1990) 655. [3] J. George, K. Ramamurthy, K.T. Varghese, S. Thomas, Appl. Polym. Sci., Part B: Polym. Phys. 38 (2000) 1104. [4] K.T. Varghese, P.P. De, G.B. Nando, S.K. De, Plast. Rubber Process. Appl. 7 (1987) 11. [5] C.S. Ha, J. Appl. Polym. Sci. 35 (1998) 2211. [6] F.S. Liao, A.C. Su, T.C.J. Asu, Polymer 35 (1994) 2579. [7] Y. Germain, B. Ernst, O. Genelot, K. Dhamani, J. Rheol. 38 (1994) 683. [8] L.H. Chu, S.H. Guo, W.Y. Chiu, H.C. Tseng, J. Appl. Polym. Sci. 49 (1993) 791. [9] S.H. Spiegelberg, A.S. Argon, R.E. Cohen, J. Appl. Polym. Sci. 53 (1994) 1251. [10] C.E. Locke, D.R. Paul, J. Appl. Polym. Sci. 17 (1973) 2791. [11] K. Min, J.L. White, J.F. Fillers, J. Appl. Polym. Sci. 29 (1984) 217. [12] R.I. Tanner, J. Polym. Sci., Part A – Z 14 (1970) 2067. [13] C.D. Han, Rheology in Polymer Processing, Academic Press, New York, 1976, p. 115. [14] T. Arai, H. Aoyarna, Trans. Soc. Rheol. 7 (1963) 333.

276

S. Joseph et al. / Materials Letters 53 (2002) 268–276

[15] R. Asaletha, G. Groeninckx, M.G. Kumaran, S. Thomas, J. Appl. Polym. Sci. 69 (1998) 2673. [16] A.Y. Coran, A.K. Bhowmick, H.L. Stephens (Eds.), Handbook of Elastomers: New Development and Technology, Marcel Dekker, New York, 1988, p. 249.

[17] S. George, K. Ramamurthy, J.S. Anand, G. Groeninckx, K.T. Varughese, S. Thomas, Polymer 40 (1999) 4325. [18] L.A. Gottler, J.R. Richwine, F.J. Wille, Rubber Chem. Technol. 55 (1982) 1448. [19] J. Tordella, J. Appl. Polym. Sci. 7 (1963) 215.