Polymer Testing 24 (2005) 241–243 www.elsevier.com/locate/polytest
Thermoplastic copolyether ester elastomer toughened polycarbonate blends 3. Microhardness and abrasion resistance of the blends P. Sivaraman, N.R. Manoj, V.S. Mishra, R.D. Raut, A.B. Samui, B.C. Chakraborty* Naval Materials Research Laboratory (D.R.D.O.), Anand Nagar, Ambernath 421506, Maharashtra, India Received 8 June 2004; accepted 19 July 2004
Abstract Blends of polycarbonate (PC) and thermoplastic copolyether ester elastomer (COPE) were prepared by melt blending using a single screw extruder in different weight percentages. Vickers microhardness and abrasion resistance measurements were carried out to study the effect of COPE composition on PC/COPE blends. It is observed that both microhardness and the abrasion resistance decrease with increase in the COPE weight percentage in the blends. The change in morphology to cocontinuous phase from droplets as the COPE content reaches 20 wt% has a significant effect on the properties. q 2004 Elsevier Ltd. All rights reserved. Keywords : Polycarbonate; Copolyether ester; Blend; Abrasion; Microhardness
1. Introduction The modification of a polymeric material by adding one or more other polymers is highly economical to achieve superior performance instead of synthesizing new materials. Among the many improvements for the commercialization of blends, the enhancement of toughness, processability, thermal stability and good weatherability are some of the important objectives. Polycarbonate (PC) is an important engineering thermoplastic and because of its excellent clarity, high heat deflection temperature and toughness, it is used in a wide range of applications. In recent years, blending of PC with a variety of polymers has attracted considerable attention as a way to improving the properties as well as to reduce the processing temperature of PC. Thermoplastic elastomers have been blended with PC for toughness improvement and better processability [1–5]. The properties of PC and
thermoplastic copolyether ester elastomer (COPE) blends have been reported for their compositional dependence on mechanical and thermal properties and their correlation with the morphology [4,5]. Microhardness testing has emerged in the recent past as a technique capable of detecting structural and morphological changes in polymeric materials [6–9]. Moreover, it has proved to be very popular for the study of polymers because of its simplicity and non-destructive nature. Hardness of the polymeric materials has a significant effect on the wear characteristics. Abrasion is one of the many types of wear where there is a displacement of material from the surface during the relative motion against hard particles. Both hardness and abrasion resistance are profoundly affected by the nature and amount of blending as well as additives. The aim of present investigation is to study the microhardness and abrasion resistance of PC/COPE blends. 2. Experimental
* Corresponding author. Tel.: C91-251-262-0608; fax: C91251-262-0604. E-mail address: [email protected]
(B.C. Chakraborty). 0142-9418/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2004.07.006
Polycarbonate (Lexan, bisphenol A type resin) used in this study was procured from GE Plastics, India.
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Thermoplastic copolyether ester, COPE (Hytrel 4069) was obtained from DuPont, India. A Brabender (Lab Station, Germany) single screw extruder was used for the melt blending of PC and COPE. The detailed procedure of blending is given elsewhere . The PC/COPE blend compositions were 100/0, 90/10, 80/20, 70/30, 60/40 and 0/ 100 in weight ratios and were coded as PC/COPE0, PC/ COPE10, PC/COPE20, PC/COPE30, PC/COPE40 and PC/ COPE100, respectively. The extrudate was obtained as 0.5G.0.1 mm thick sheets and were subsequently compression molded at 250 8C in a molding press at 20 MPa pressure to get sheets of thickness 6.35G0.05 mm. Test specimens for microhardness and abrasion tests were cut from these sheets. Vickers microhardness test was carried out using a LECO (model LM700AT, USA) microhardness tester with a microscope attached. The blend samples were indented with a Vickers diamond pyramidal indenter having a square base and 1368 pyramidal angle. The load was varied from 10 to 100 g and several indentations were made for each load and the average hardness values were taken for the analysis. Abrasion resistance of the blends was measured using an Erichsen Abrasion test apparatus Model 533 (Germany). The instrument consists of a wheel of diameter 50 mm and width of 13 mm, and is covered with the abrasive paper (specification: J297). During the test, the wheel moves to and fro in a straight line across the polymer surface with a particular load. The abrasion movement is 30 mm at the rate of 50 to and fro motions per minute, applying a force of 5 N. The abrasion wheel is raised and turned after each cycle so that for each to and fro motion a new section of paper comes into action. The abrasion resistance is reported as the weight loss in milligrams for 1000 cycles (to and fro).
3. Result and discussion It has been observed in the earlier study that the blend containing 20 wt% COPE gives the maximum mechanical properties and impact strength and this has been attributed to the development of characteristic blend morphology . DSC and DMA studies indicated a partial miscibility, which has been attributed to the migration of PC oligomers present in PC, or generated while mixing, into the COPE phase .
Fig. 1. Microhardness of PC/COPE blends.
with increase in the load and reaches almost saturation value at higher loads. The increase in the hardness can be attributed to the strain-hardening phenomenon. On increasing the load, the strain hardening increases. As the microhardness value tends to reach a constant, the polymer is completely strain hardened [7,8]. The addition of COPE to PC increases the strain hardening in the blends . It can be also seen that pure PC has higher hardness than the blends. Increasing the COPE weight percentage decreases the hardness of the blends and increases its flexibility, by acting as a softener. Another interesting observation is that there is a large gap in the microhardness values between hardness curves of the blends PC/COPE20 and PC/COPE30. In other words, there is sudden decrease in the microhardness values from PC/COPE20 to PC/COPE30 throughout the range of load studied. This can be attributed to the morphological changes in the blends. Materials with greater resistance to elastic and/or plastic deformation are usually harder because one or both of the deformation zones are not so large, and a smaller indentation results [10,11]. Morphological changes in
3.1. Microhardness Hardness is the resistance of a body to local deformation by another body. When the indenter penetrates the material, two zones of deformation are formed in the material—a zone of severe plastic deformation surrounded by a larger zone of elastic deformation. The force exerted by the indenter is supported by these two zones [10,11]. Fig. 1 illustrates the microhardness of PC and PC/COPE blends as a function of load. From the figure it is evident that the hardness for the pure PC as well as for the blends increases
Fig. 2. Abrasion resistance of PC/COPE blends.
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the polymers greatly affect the microhardness [9,12,13]. Two-phase morphology in blends, i.e. when the two components exist as finely dispersed or finely divided phases, may increase the hardness since the indentation motion may be restricted by the elastic interactions of the two phases. After 20 wt%, the PC/COPE blends changes the morphology from discrete droplets to co-continuous phase . 3.2. Abrasion resistance Fig. 2 illustrates the effect of COPE on the abrasion resistance of PC/COPE blends. The addition of COPE in the blend decreases the abrasion resistance of the blends. As the hardness of the blends decreases, the abrasion resistance also decreases, as expected. The frictional force acting on the abrading surface is the sum of two types of action—the localized forces of adhesion being overcome by shear and interpenetrating asperities giving rise to further deformation . In rigid polymers like PC, the frictional work is dissipated over a narrow interfacial zone via the rupture of adhesive bonds such as the van der Waals type or some polar interactions. Once a viscoelastic phase like COPE is included, any deformation will lead to microcutting and consequent tear, cracking and fatigue, thereby increasing the abrasion loss . The abrasion loss in case of PC/COPE20 is limited as compared to others because of the COPE phase present as droplets preventing dissipation of frictional force over a large area. The blends PC/COPE30 and PC/COPE40 show much higher abrasion than the blends PC/COPE10 and PC/COPE20. This can be explained on the basis of morphological changes due to the increase COPE content in the PC/COPE blends. In the PC/COPE10 and PC/COPE20 blends, PC is the continuous phase and COPE is the discrete phase, whereas in PC/COPE30 and PC/COPE40 blends both PC and COPE form co-continuous phases . In the co-continuous phases, the frictional forces are not dissipated
easily as in finely dispersed phases, leading to deformations and consequent microcracking.
4. Conclusion The microhardness and the abrasion resistance of the PC/COPE blends were studied and found to decrease with increase in the COPE content. The morphology of the blends has a pronounced effect on the microhardness and the abrasion resistance.
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