55 Acta Chim. Slov. 1999, 46(1), pp. 55-67 THE INFLUENCE OF THERMOPLASTIC ELASTOMERS ON MORPHOLOGICAL AND MECHANICAL PROPERTIES OF PP/TALC COMPOSITES1 Matjaž Denac1, Vojko Musil2 1) University of Maribor, Faculty of Chemistry and Chemical Engineering, Maribor 2) University of Maribor, EPF Maribor, Institute of Technology (Received 17.12.1998) Abstract: Recent investigations have shown that modification of polymer matrix with filler and elastomers significantly affects composite’s mechanical properties. Isotactic PP modified with either untreated or treated talc and either SEBS or SEBS-gMA were used in these investigations. Samples were prepared by melt-mixing in a Brabender kneading chamber and were compression molded into plates on a laboratory press. The composites were characterised by measuring mechanical properties (Young’s modulus, yield stress, notched impact strength) and by defining morphology. Binary sistems PP/talc and PP/elastomer containing up to 16 vol.% of talc and up to 10 vol.% of elastomer, as well as ternary PP/talc/elastomer composites with 12 vol.% talc were investigated. Ternary composite’s yield stress was also calculated b semiempirical equations. We have found out, that the use of treated modifiers (talc or elastomer) improves adhesion with matrix, which reflects on mechanical properties as better stress transfer. Introduction An important recent development in polymeric materials concerns the preparation of hybrid particulate composites obtained by incorporating both rubbery and inorganic particles into a polymer matrix. The addition of the elastomeric phase enhances the material toughness, whereas the addition of inorganic fillers increases the material stiffness. Combination of filler and elastomer compensate either elasticity or strength reduction which can rise by introducing in PP matrix only one component (either talc or elastomer). Moreover, Dedicated to the memory of Professor Dr. Anton Šebenik 56 addition of both components can offer other advantages such as reduction in cost, a good surface appearance, and improved processing behaviour [1]. Among thermoplastics polypropylene (PP) is outstanding with respect to its attractive price/performance ratio combined with heat distortion temperature above 100oC, and high stiffness. Poor adhesion between filler and PP is a primary cause of low strength and poor thermomechanical properties, especialy at high filler volume fractions [2]. Therefore, PP is frequently toughned by incorporationg elastomer microparticles, e.g. by blending of PP with elastomers [3,4]. This paper describes the influence of thermoplastic elastomers (TPE-s) on morphology development and mechanical properties, e.g. stiffness, strength, and toughness of talc-reinforced PP. Two thermoplastic elastomers were used, block poly-(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) and the corresponding block copolymer grafted with maleic anhydride (SEBS-gMA). Pure or aminosilane-functionalized talc was blended together with PP in a Brab ender kneader. Morphology and mechanical properties were studied as a function both type and volume fraction of TPE. The comparison of theoretical and measured values of mechanical properties is discused. Experimental Materials For preparation of binary PP/talc and PP/ elastomer systems, as well as ternary PP/talc/elastomer composites, the folowing materials were used: • polypropylene (PP) Novolen 1100 L (BASF), MFI=6g/10min, p=0.908g/cm3, Mn=47000*, Mw/Mn=9.3* • untreated talc, Talk Naintsch A-20 (Luzenac), particle size (top cut)= 20|im, p=2.8g/cm3, specific surface = 24000 2/g • talc with aminosilane surface treated (2%), Talk Naintsch A-20 V592 (Luzenac), particle size (top cut)= 20|im, p=2.8g/cm3. specific surface = 24000 2/g • block poly-(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) Kraton G-1650 (Shell), styrene/rubber ratio=28/72, p=0.91g/cm3, Mn=92400*, Mw/Mn=1.19* 57 • block poly-(styrene-b-ethylene-co-butylene-b-styrene) grafted wit maleic anhydrid (SEBS-gMA) Kraton KG-1901 (Shell), styrene/rubber ratio=29/71, p=0.91g/cm3, Mn=47300*, Mw/Mn=1.55* (*/ Values were determined by size exclusion chromatography.) Sample preparation Binary PP/talc and PP/elastomer systems with talc ratio 4, 8, 12 and 16 vol.% and elastomer ratio 2.5, 5 and 10 vol.%, as well as ternary PP/untreated talc/elastomer and PP/treated talc/elastomer composites with 12 vol.% of talc and elastomer content of 2.5, 5 and 10 vol.% were prepared in a Brabender kneading chamber. Into a chamber a 200oC and rotor speed 50 min -1 first the filler was added and after 30s PP with elastomer. After that, the samples were kneaded for 6 more minutes. The melt was pressed into 1 and 4 mm plates on laboratory press. The pressing temperature was 220oC, pressure 100 bar, pressing time 14 min for 1mm and 9.5 min for 4 mm thick plates. Plates were cooled to room temperature in the air. Test methods Tensile properties (Young’s modulus, yield stress) were measured by Zwick 147670 Z100/SN5A apparatus at 23°C and strain rate of 2mm/min (ISO 527). Notched impact strength was measured according to Charpy test (DIN 53453) by Zwick apparatus at 25°C. Scanning electron microscope (SEM) Jeol JSM 840-A was used to study the morphology, magnification was 2000- and 9000-times at acceleration voltage 10kV. Samples were fractured in liquid nitrogen and covered with gold before examination by the microscope. Results and discussion Young’s modulus of PP hybrid composites containing both talc and elastomer, measured as slope of strain-stress curve, determined as a function o elastomer type and volume fraction (cpe), is depicted in Fig.1. As expected, the hybrid composite moduli decreased with increasing elastomer volume fraction. Values of Young’s moduli of PP 58 hybrid composites containing SEBS are higher than that of corresponding hybrid composites containing SEBS-gMA. Figure 1: Young's modulus of pure PP and modified PP/treated talc composites as a function of elastomer volume fraction at talc content 12 vol.% In previous work [5] it was shown that in PP/talc/SEBS systems SEBS is dispersed as separate phase, while at PP hybrid composite with treated talc, SEBS- gMA can encapsulate talc particles to form core-shell particles consistin g of t alc core and SEBS-gMA shell (Fig.2). The size of dispersed particles depends on SEBS molecular weight and on SEBS content. Figure 2: Morphologies of PP hybrid composites: a) three phase morphology with separately dispersed elastomer and filler particles b) two phase morphology with core-shell particles (filler particle core and elastomeric shell) 59 At binary PP/talc composites, talc particles significantly increase stiffness and at filler volume fraction 12 % practically don’t affect the tensile strength at yield [6]. In ternary composites with SEBS-gMA, the addition of talc, as elastomer subinclusion, should decrease volume fraction of PP and contribute to form thin interlayer o elastomer. Matonis and Small [7] proposed that very thin elastomeric interlayers with thickness of about 1/1000 of particle diameter between filler and matrix increase Young’s modulus, whereas thicker surface interlayers decrease modulus. From Fig.1 it is obvious that thicker interlayers of elastomeric shells are formed, which is related to molecular weights of used elastomers and size distribution of talc particles [8]. SEM studies of cryo-fractured surfaces of PP/treated talc/ elastomer composites are presented in Figs.3 (a-e). SEM images of fractured surfaces of PP hybrid composites, containing 12 vol.% talc and 5 vol.% SEBS or SEBS-gMA, show sufficient adhesion between filler and matrix. SEBS is randomly dispersed in matrix, higher volume fraction is reflected as a strong coalescence. In case of using up to 5 vol.% SEBS-gMA, the elastomer particles are located on talc surface and the surface is very difuse, owing to quality of interfacial adhesion. At 10 vol.% of SEBS- gMA first core-shell is observed, the talc surface is less difuse and SEBS-gMA particles are present. From SEM images it is not clear wheather these particles are also core-shell formations or particles of pure SEBS-gMA. Tensile yield stress, determined at large deformations, is an excellent measure for interfacial interactions in heterogeneous polymer systems [9,10]. With the help of the theory of tensile yield stress it is possible to determine a parameter related to stress transfer of blends and filled polymers. Pukanszky [11] derived an equation for blends and composites with one single dispersed phase: ay =(Jy0-------------• exp (Bçd) (1) where oy is the yield stress of the blend and composite, respectively, and oyo the matrix yield stress. Çd is the volume fraction of the dispersed phase. 60 Figure 3: Scanning electron micrographs of hybrid composites; a) surface of unextracted PP with 12 vol.% treated talc and 5 vol.% SEBS, b) surface of xylene extracted PP with 12 vol.% treated talc and 5 vol.% SEBS, c) surface of unextracted PP with 12 vol.% treated talc and 5 vol.% SEBS-gMA, d) surface of xylene extracted PP with 12 vol.% treated talc and 5 vol.% SEBS-gMA, e) surface of xylene extracted PP with 12 vol.% treated talc and 10 vol.% SEBS-gMA The second factor takes into account the smaller effective load-bearing cross section by replacing matrix polymer by dispersed phase. Parameter B in the third factor considers stress transfer between dispersed phase and matrix and indicates irregular structures or processes (aggregation, orientation of anisotropic filler particles), what can considerably influence the yield stress. Strong interfacial interactions lead to high values of B and consequently to high yield stress of corresponding system. Higher specific surface and anisotropy of filler is reflected in higher values of parameter B, while aglomeration and surface treatment of filler have an opposite effect. Parameter B can be evaluated from experimental data in form o linearized graph. There are two possibilities of linearisation. The first one assumes a tensile yield stress o matrix (sy0) to be constant and equation can be linearized as follows: 61 ln ( oy 1 + 2.5^ y 0 1^: = lno", ) y (rei) ~ ^Wd (2) If lnOy(rei)* is plotted against volume fraction of dispersed phase, parameter B can be calculated as a line slope, with intercept in cross section of coordinate axes. The second approach takes into account the fact that we cannot keep oyo constant, so linear correlation gives equation (3), where B is a slope and oyo can be calculated fro intercept. ln o-{1 + 25(pa ^P~d = B(pd + lna} y0 (3) In Fig.4 lnöy(rei.)* is plotted against volume fraction of dispersed phase (talc or SEBS-gMA) according to equation (2) and parameters B are determined. In Fig.5 parameters B are determined by equation (3). 0,600 - o 0,500 - / 0,400 - '' /o o measured T592 I gl 0,300 - ---------calculated T592 A measured SEBS-gMA C /o ---------calculated SEBS-gMA | 0,200 - 0,100 - /o > 0,000 t / -'' ^ ( } 0,05 0,1