morphology studies and mechanical properties for ps/sbs blends · thermoplastic polymer represents...

International Journal of Engineering & Technology IJET-IJENS Vol: 12 No: 03 19

120903-9494 IJET-IJENS @ June 2012 IJENS I J E N S

Morphology Studies and Mechanical Properties for PS/SBS Blends

Buthaina A. Ibrahim* and Karrer M. Kadum**

*University of Technology, Department of Applied Sciences, Iraq-Baghdad,

**Ministry of science and technology Iraq-Karbala

Email: * [email protected] ** [email protected]

ABSTRACT

Polymer blending is of growing importance nowadays, because the blend can be tailored to meet the requirements of specific applications. Blends of Polystyrene (PS) and Styrene-Butadiene-Styrene (SBS), as thermoplastic elastomers are prepared in different ratios by melt blending technique using Haake Poly-Driver extruder. Tensile test, Differential Scanning Calorimetry (DSC), and Scanning Electron Microscopy (SEM) are used to study the mechanical and thermal properties and morphology.

The test results show that the mechanical properties depend on the SBS content, i.e. the tensile strength and Young's modulus are found to decrease with increasing SBS composition, while ultimate elongation and impact strength are found to increase with increasing SBS composition. Test results show that all mechanical properties nearly follow the rule-of-mixture behavior. The DSC test results confirm state of miscibility for all blend ratios. The SEM results fully support those results, obtained from mechanical properties.

KEYWORDS: Polystyrene (PS), Styrene-Butadiene- Styrene (SBS), Polymer Blends, Morphology, Mechanical Properties.

1. INTRODUCTION

Blending is a convenient and practical technique for developing materials whose properties may be novel or selectively enhanced, which are possibly superior to those of the components [1-3].

Polystyrene (PS) is an excellent engineering thermoplastic material, due to its clarity, hardness and low cost. Polystyrene (PS) use is limited, due to its brittleness, poor processibility and chemical resistance at room temperature [4-5].

Polymer and copolymer blending is a viable method to formulate polymeric materials with enhanced stiffness and toughness, such as ABS, SEBS and SBS [6-11]. The combination of elastomer with thermoplastic polymer represents an important group of blends [12 ,13]. Styrene-Butadiene-Styrene triblock copolymer (SBS) is a thermoplastic elastomer, which has better impact strength and thermal stability than other types of rubbers; it has high processing temperature and due to its lower cost is most commonly used with thermoplastics.

Melt blending is an attractive method to modify polymers, being simple, fast, effective and economical way in improving the disadvantages of raw materials [14]. It has been reported that binary system of HIPS/SBS shows best mechanical properties for (90/10) % blends [15].



In previous studies of PS/ABS (Acrylonitrile-Butadiene-Styrene) blend, it was shown that it possesses better mechanical properties for ratios (70/30, 60/40, and 50/50) w/% than pure polymers [16]. A series of experiments have been carried out in this work on various PS/SBS blend ratios, aimed at clarifying how thermoplastic elastomer improves the PS’s mechanical properties.

2. Materials and Test Methods

PS and SBS are obtained from the Chemical Industries Company, Baghdad. Mixtures of the two polymers were prepared with different ratios of (100, 90, 80, 70, 60, 50, 40, 30, 20, 10 and 0) w/0 compositions of (PS/SBS).

Blending was achieved by melt extrusion using Haake Poly-Drive extruder. The extruder operational conditions used were 50 rpm with temperature of front, mix chamber and near panel zone at 200 oC. Then the blends were molded by using hot press machine at temperature 20 oC and 10-ton pressure.

Testing Methods

The tensile testing was carried out for all prepared specimens in accordance with ASTM D638 Standard, using Instron testing machine (Model PL TE-32-TSQ) with cross-head speed of 0.5 mm/min.. The load applied was within the range (0-5000)N.

The Izod Impact testing was performed according to ISO-179 Standard using Zwick 5113 Pendulum Impact Tester Model IMI.

The DSC measurements of glass-transition temperature were performed using a Perkin Elmer Differential Scanning Calorimeter Model (DSC 60) with approximately 20mgm. Each sample was scanned from initial temperature (-70oC) at a rate of 10oC/min.. Liquid Nitrogen was used as a cooling system for measurement of Tg below room temperature.

Scanning Electron Microscope (SEM) model (Karl Zeiss EVO 50) Japan was used to investigate the morphology of pure and blend specimens. In order to investigate the internal structure, the specimens were submerged in liquid nitrogen to ensure peeling is performed, while the sample was frozen.

3. Results and Discussion

a- Mechanical Properties

To obtain a stress-strain curve for PS homo-polymer proved very difficult; this is attributed to its high brittleness. The mechanical behavior test of thermoplastic elastomer (BS) and PS in the blend are shown in Figures (1-6) for the selected compositions i.e. 10, 30, 50, 70, 90 SBS wt% respectively.



Figure (1): stress-strain curve for (10/90) SBS/PS composition Figure (2): stress-strain curve for (30/70) SBS/PS composition

Figure (3): stress-strain curve for (50/50) SBS/PS composition Figure (4): stress-strain curve for (70/30) SBS/PS composition

Figure (5): stress-strain curve for (90/10) SBS/PS composition Figure (6): stress-strain curve for pure SBS



Young's modulus, yield strength and strength at break are found to decrease dramatically with increasing SBS content especially for the 10 and 20% SBS compositions, as shown in Figures (7, 8). On the other hand, the incorporation of a rubber in the blend increases the ultimate strength as shown in Figure (9).

Furthermore, the results of Izod impact test results are shown in Figure (10). The recorded results show that with increasing SBS content, the impact strength improves from 20J/m for pure PS to 201J/m for 60 wt% SBS.

Fig (7): Effect of SBS content on Young’s modulus Fig(8): Yield strength and Break strength as a function of SBS content.

Fig (9): Effect of SBS content on Elongation at break. Fig (10): Effect of SBS content on Impact resistance

As a general trend, the addition of small proportions of SBS to PS, i.e. less than 30%, the material is found hard and strong, but not as brittle as pure PS. The addition of large proportions of SBS tend to make the material ductile and elastic, which enhances the rubber-like behavior, but stronger than pure SBS. The compositions, from 30 to 70 w% of SBS, show that mechanical properties change linearly with composition, which may suggest obeying the rule of mixture [17,18].



From the results of mechanical behavior of heterogeneous systems in different situations, it may be concluded

that they depend mainly on their compositions, such as stiff-phase PS dispersed in soft matrix (as for high

composition of SBS), which behave like rubber-like materials or soft phase SBS dispersed in stiff matrix (as for

high composition of PS), which behave like hard and brittle materials. The same observations and results have

been reported by other researchers [19,20].

b- DSC Test

The DSC results, for pure polymers and their blends, are summarized in Table 1

Table 1

PS/SBS (w/%) Composition Tg1

oC Tg2 oC

100/0 ------- 116.0

90/10 -64 112.6

80/20 -63 111.1

60/40 -60 109.1

40/60 -58 94.0

20/80 -58 90.0

0/100 -64 78.7

From the results, it is observed that the DSC thermogram for SBS shows two glass-transitions, one for soft elastomer phase of PB block at (-64oC) and the other for hard plastic phase of PS block at (78.7oC). Pure PS exhibits only one glass transition temperature at 116oC, which is higher than that of the SBS block copolymer.

The PS/SBS blends of different compositions show also two Tg's ; the Tg1 of the butadiene phase (PB) changed very slightly upon mixing of PS with SBS, thus suggesting a negligible interaction of homo-polymer (PS) with the PB blocks. The more significant changes can be observed for the second glass-transition temperature Tg2, which decreases with increasing content of SBS in the blend, i.e. from 112.6 oC to 90 oC for 10 to 80 wt% SBS respectively. These results suggest that PS is compatible with PS blocks of SBS, and the shift in the two Tg's becomes smaller with increasing SBS proportions becomes smaller with increasing SBS proportions. DSC is used especially for discrimination between miscible and immiscible polymer blends [21, 22]. One Tg indicates a miscible system, two Tg’s coinciding with related Tg for the components indicate an immiscible blend, and two Tg’s shifted to the direction of their average is typical of partially miscible system.



c-SEM

Scanning electron micrographs for internal morphological studies of the melt mixing of pure polymer and blends of PS/SBS with different compositions are shown in Figure (11).

The increase of SBS proportions to 20%, as shown in Figure (11.d), indicates that the particle size increases considerably, but this pure phase is still isolated from the matrix phase. For 30% SBS blend, it starts to percolate forming layer domains of complicated shapes and a tendency of initiating interconnection and no more isolated particles observed, as appear very clearly in Figure (11.f), continuous phases are observed clearly with fiber with fiber connections between each phase for 50% SBS composition. When the SBS constituents are increased to 70%, the micrograph shows an intermediate co-continuity between phases and PS-phase dispersed in the continuous SBS matrix phase as shown in Figure (11.g). The mechanical properties given above for the 30% to 70% SBS blends were found to follow the rule of mixture. This may reflect the influence of co-continuous phases on mechanical properties of the blend.

Furthermore, as in Figure (11.h), the micrograph for 80% SBS shows PS domain dispersed in the continuous elastomer matrix phase. Similar behavior of two-phase morphology has been reported for PS/PB blends [24, 25].

(a) Pure PS (b) Pure PS

( c ) PS/SBS [90/10]% (d) PS/SBS[80/20]%



(e) PS/SBS [70/30]% (f) PS/SBS [50/50]%.

(g) PS/SBS [30/70] (h) PS/SBS [20/80] %. Fig (11): SEM photomicrographs for : (a) PURE PS ( b) PURE SBS (c) PS/SBS [90/10]% (d) PS/SBS[80/20]% (e) PS/SBS [70/30]% (f) PS/SBS [50/50]%. (g) PS/SBS [30/70] (h) PS/SBS [20/80] %.

Conclusions

• A polymer with desirable properties is developed by mixing the hard and brittle PS with thermoplastic elastomer SBS, at different ratios by melt blending technique.

• The mechanical properties for blend system confirm the viability of controlling the product property by controlling the blend ratios, which suggests the rule of mixture is obeyed.

• The SEM results for blend system at low proportions of SBS or PS show that the minor phase is dispersed in the continuous matrix phase, while for intermediate proportions show co-continuous phases.

• The DSC results for blend system show that Tg depends on the ratio of SBS to PS in the blend.



• The results give the impetus to project these findings for future work to investigate the effect of the environment and thermal history on the mechanical and thermal properties of different blend systems. Another issue of interest is the study of the morphology of PS/SBS blend by SAXS.

References

[1] Z. Horak, I. Fortelny, J.Kolarik, D. Hlavati and A. Sikora, "Encyclopedia of Polymer

Science and Technology", Polymer Blends, John Wiley and Sons, (2005).

[2] L. A. Utracki, "Polymer Blends Handbook", published by Kluwer Academic Publishers,

The Netherlands, (2002).

[3] O. Olabisi, L. Robeson, and M. T. Shaw, "Polymer-Polymer Miscibility", Academic

Press, New York, (1979).

[4] J. E. Gray, "Polystyrene: Properties, Performance and Applications", Nova Science

Publishers, (2011).

[5] James E. Mark, "Polymer Data Handbook", USA, Oxford University Press, (1999).

[6] M. M. Khan, R. F. Liang, R. K. Gupta and S. Agrawal, "Korea-Australia Rheology

Journal", Vol. 17, No. 1 (2005).

[7] S. A. Samsudin, A. Hassan, M. Mokhtar and S. M. S. Jamaluddin, "Malaysian Polymer

Journal", Vol. 1, No. 1, (2006).

[8] S. Joseph, W. Focke, and S. Thamas, "Composite Interfaces", Vol. (17), No. 2, (2010).

[9] S. Joseph, F. Laupretre, C. Negrell and S. Thomas, "Polymer", 46, (2005).

[10] E. V. Sanchez, J. L. Ribelles, M. M. Pradas, B. R. Figueron and F. R. Colomer,

"European Polymer Journal", 36, (2000).

[11] S. Joseph, A. R. R. Menon, Aju Joseph and S. Thomas, "J. Mater. Sci.", 42, (2007).

[12] S. Joseph, Z. Oommen and S. Thomas, "Materials Letters", 53, (2002).

[13] Fried J.R, "polymer science and technology" ,2nd edition hill, India ,(2005).

[14] G. Shearer and C. Trogunakis, "Polymer Engineering and Science", Vol. 39, Issue 9,

(1999).

[15] Z. Jelcic, T. Holjevac-Grgunic, and V. Rek, "Polymer Degradation and Stability", 90,

(2005).

[16] B. A. Ibrahim and K. M. Kadum, "Modern Applied Science", Vol. 4, No. 9, (2010).



[17] W. Viratyaporn, R. L. Lehman, and J. Joshi, "Society of Plastic Engineers [SPE]

Annual Technical Conference Proceedings, Cincinna", OH May, (2007).

[18]Azman Hassan and Wong Yean Jwu, “Simposium Polimer Kebangsaan” , P.p.(65–76),

(2005).

[19] T. Kotaka, T. Miki and K. Arai, J. of Macromolecular Science, Part B: Physics, Vol.17,

Issue 2,

(1980)

[20] T. A. Huy, R. Adhikari, Th. Lupke, G. H. Michler and K. Knoll, Polym. Eng. Sci. Vol.

44, 1534-1542, (2004).

[21] W. J. Macknight, F.E. Karasz and J. R. Fried, “Polymer Blends”, Academic Press, Inc.,

New York, 1978, Ch. 5.

[22] F. Picchioni, E. Passaglia, G. Ruggeri and F. Ciardelli, Macromol. Chem. Phys. Vol.

202, 2142- 2147 (2001).

[23] B. Brahimi, A. Ait-Kadi, A. Ajji and R. Fayt, J. Poly. Sci. Part B: Polymer Phys. Vol.

29, 945 (1991).

[24] S. Joseph and S, Thomas, "European polymer Journal", Vol. 39, (2003).

[25] I. Fortelny and D. Michalkova, Plast. Rubber Compos. Process, Appl., Vol. 27 (2), 53

(1998).

morphology studies and mechanical properties for ps/sbs blends · thermoplastic polymer represents...

Documents