Properties of polypropylene modified with elastomers

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1 Plasticheskie Massy, No. 5, 2005, pp Properties of polypropylene modified with elastomers G. M. Danilova-Volkovskaya Rostov State Academy of Agricultural Engineering Selected from International Polymer Science and Technology, 32, No. 8, 2005, reference PM 05/05/31; transl. serial no Translation submitted by P. Curtis The aim of the investigation was to improve the processing properties of PP, and the strength and deformation properties of articles manufactured from it, by introducing modifying additives during processing. The basis for selecting the additives was a study of their effect on the rheological, relaxation, and deformation and strength properties of the polymer and the final properties of articles manufactured from it [1]. In most cases, the dependence of the physicomechanical properties on the concentration of the elastomer additive is extremal in nature. The position of the maximum on the composition property dependence is determined by the type of polymer and the form of modifier. Elastomer-modified polyolefins possess greater resistance to repeated alternating bending, impact loads, and considerable cracking resistance in a medium of surfactants [2, 3]. The reduction in melt viscosity of the materials increases the output of the process and reduces the wear of the equipment. The modifying additives used were a whole series of substances of different nature and molecular weight that are being produced by industry butadiene styrene rubbers (SKS, SKS-30, and SKS-50) and thermoplastic elastomers (piperylene butadiene styrene thermoplastic elastomer PBSTR, divinylstyrene thermoplastic elastomers of linear and branched structure DST-30 and DST-30R). The effectiveness of modification of PP is influenced by the intensity and conditions of thermomechanical action during processing, which can be controlled under conditions of twin-screw and screw-disc extrusion, varying the shear rate and stress and the magnitude of strain of the melt, creating a high level of dispersing and homogenising mixing [4]. The content of structures in the modifiers before and after modification was determined by methods of qualitative and quantitative analysis of polymeric materials: methods of gas plasma chromatography for qualitative elemental analysis of the concentrate and X-ray spectral analysis for quantitative elemental analysis on a CAMIBAX unit. The data are presented in Table 1. From the dependences of the tensile strength of specimens of PP modified during screw-disc extrusion on the parameters of processing (Figure 1) and the amount of modifier introduced, it can be concluded that they are all extremal in nature, i.e. there are optimum regimes and a critical concentration of the additive with which specimens of modified material possess the highest values of strength (Figure 2). Table 1. Structural composition of elastomeric modifiers Content of Modifier structures, % SKS SKS-30 SKS-50 DST-30 DST-30R 1,4-cis ,4-trans ,2-trans Rapra Technology Limited T/21

2 Figure 1. Dependence of tensile strength of specimens of modified PP on processing temperature (1, 2, 3) and shear rate (1, 2, 3 ) for modification of PP with butadiene styrene rubbers SKS, SKS-30, and SKS-50 and thermoplastic elastomers DST-30, DST-30R, PBSTR, etc., with variation in their content from 2 to 20% Figure 2. Dependence of tensile strength on content of modifiers: 1 SKS-30; 2 DST; 3 SKS-30; 4 DST-30 Figure 3. Dependence of specific work of impact strength on content of DST-30 Figure 4. Dependence of effectiveness of copolymerisation of PP and DST-30R on percentage content (left) and shear rate during processing (right) For elastomer additives this content mainly fluctuates from 5 to 15 wt.%. High strength values are possessed by specimens of PP modified with DST-30R, SKS, and SKS-30. Increase in the indices of the specific work of impact strength (Figure 3) and deformation and strength characteristics during ageing in the presence of modifiers was also confirmed. The modification of PP during screw-disc extrusion was carried out with a temperature of the disc zone of 190 C, the choice of which, as shown by preliminary investigations, was governed by the maximum intensity of mechanochemical processes in the polypropylene matrix [5]. The structure of the formed copolymers, produced by thermomechanical modification of the PP with the elastomer in the process of their combined processing, depends on the properties of the components and the process conditions. Since modification occurs in the presence of air oxygen and macromolecules of PP and TPE are capable of degrading and crosslinking under intense thermomechanical action, copolymers of very complex structure are formed, and here the composition of the formed products is made considerably more complex, since segments of block and graft copolymers themselves may undergo mechanodegradation with the formation of free radicals. As a result of such thermomechanochemical modification, a mixture of multicomponent graft and block copolymers is formed. The identification of such products is extremely difficult, and therefore it is possible to provide only certain proof T/22 International Polymer Science and Technology, Vol. 33, No. 1, 2006

3 of the formation of graft and block copolymers and to study the infl uence of their formation on the properties of the modified polymer. The effectiveness of interaction of the components was assessed from the amount of elastomer remaining in the modified PP after selective extraction in carbon tetrachloride in a Soxhlet apparatus for 30 h in the presence of an antioxidant for prevention of processes of additional structure formation. The dependence of the amount of modifier remaining on the conditions of its introduction showed that, with increase in the content of modifier to 10 wt.%, the percentage content of combined TPE increases from 50 to 82% of the amount introduced. With further increase in the content of TPE introduced, the effectiveness of interaction of the components falls. This effect is evidently connected with a certain plasticising effect rendered by uncombined thermoplastic elastomer and products of its degradation, which prevents chemical interaction of the components (Figure 4). From an analysis of the dependence of the quantitative ratio of graft and free modifier on the shear rate during processing, the following conclusions can be drawn: at low shear rates, the grafting process is not able to be realised, and its activation is observed only on the achievement of a shear rate of 250 s 1 ; with increase in the shear rate to s 1, the amount of modifier combined with PP falls, which may be connected with the predominance of processes of degradation of the polymer and additive. The modification products were studied by Fourier IR spectroscopy. Investigation of chemically modified PP subjected to extraction by IR spectroscopy in the frequency range cm 1 showed the appearance in the IR spectra of characteristic bands belonging to the thermoplastic elastomer: the 540, 560, 700, and 760 cm 1 bands correspond to vibrations of the styrene part of the TPE, the 967 cm 1 band corresponds to 1,4-trans-isomers, the 740 and 100 cm 1 bands correspond to 1,4-cis-isomers, and the 998 cm 1 band corresponds to 1,2-isomers of butadiene. In specimens of PP not subjected to selective extraction, retention of the given bands and increase in their optical density was observed. In the spectra of PP modified at high shear rates, compared with spectra of the initial PP and PP modified at low shear rates, where appreciable grafting is ruled out, a sharp increase is observed in the intensity of the bands corresponding to vibrations of ester and ether groups, in particular at 1030, 1050, and 1070 cm 1 (ether groups), 1240 and 1260 cm 1 (ester groups), and carbonyl groups determined from the 1720 cm 1 band. This is connected with the mechanism of oxidation of products of thermomechanochemical degradation of components proceeding in the diene part of the modifier, and also, possibly, the formation of degradation products of PP. The presence of oxygen-containing groups was also established in IR spectra of the sol fraction of degradation products of thermoplastic elastomers that did not react with PP during thermomechanochemical synthesis. With increase in the shear rate during modification there is an increase in the relative number of ruptures of the macromolecules and, consequently, in the probability of their oxidation by oxygen, whch is confirmed by an increase in the number of carbonyl groups with increase in the shear rate and in the amount of unreacted modifier also prone to oxidation. In extracted specimens of modified PP, the appearance of a 1360 cm 1 band characterising vibrations of the quaternary carbon atom is fixed, which indicates the occurrence of processes of branching in the macromolecules. Thus, IR spectroscopy confi rmed that, during the thermomechanochemical modification of PP with elastomer, the formation of block and graft copolymers occurs, and the degradation products of the thermoplastic elastomer, not combined with the PP, also have a complex composition, are prone to oxidation, and have a considerable infl uence on the structure and properties of the chemically modified PP. Primary assessment of the effect of additives on the viscous properties of the PP melt was carried out by measuring the melt fl ow index (MFI) of the specimens. From the form of the curves of the dependences of the MFI on the quantitative content of modifier it can be concluded that the presence of elastomeric additives lowers the MFI; the reverse effect is observed only in the presence of SKTN, SKS, and DST-30R. To obtain complete information on the infl uence of processing parameters on the properties of PP in the presence of modifying additives, an investigation was made of the rheological and relaxation properties of the polymer system when intense shear effects are imposed. The results of the investigations were interpreted using specially developed procedures [5]. The rheological and relaxation properties of melts of the materials were studied on an Instron-3211 capillary viscometer in a wide temperature and shear stress range, with a shear rate of s 1. Initially, in the plotting of fl ow curves, account was taken of entry corrections, which were calculated by the Begley method (the two-capillary method). However, the magnitude of the entry corrections amounted to less than 5% of the entry pressure, which makes it possible to disregard them in the plotting of the fl ow curves of material melts Rapra Technology Limited T/23

4 Figure 5. Dependence of effective viscosity on shear rate in viscometric tests of PP modified with different amounts of DST-30R: 1 initial PP; 2 20%; 3 15%; 4 10%; 5 5%; 6 2% Figure 6. Relaxation spectra of melts of PP modified with different amounts of DST-30R: 1 initial PP; 2 20%; 3 15%; 4 10%; 5 5%; 6 2% Reduction in the effective viscosity of PP melts is evidently connected with the presence of small quantities of TPE and products of its degradation that are not combined with the PP and therefore have an active influence on the rheological properties of the modified material. This effect is confirmed by the displacement of curves of the relaxation spectrum towards shorter relaxation times, which indicates the acceleration of relaxation processes in the modified material (Figure 6). Investigations of stress relaxation under conditions of application of a compressive load showed that the observed effect of acceleration of relaxation processes also appears with the given type of loading. The smallest equilibrium stresses are possessed by the PP + 10%DST- 30 and PP + 8%SKS-30 systems at all the strain values investigated. With increase in the modifier content, increase in the rate of relaxation processes was not observed. The greatest reduction in effective viscosity of the material was recorded in the presence of 12 wt.% DST-30R and 10 wt.% SKS. During the thermomechanochemical modification of PP with divinylstyrene thermoplastic elastomer with a shear rate of 250 s 1 and a temperature of 190 C, and with variation in the elastomer content from 2 to 20 wt.%, displacement of the curves of the dependence of the effective viscosity on the shear stress is observed towards lower shear stresses with an elastomer content of 2 10 wt.% and towards greater shear stresses with an increase in the elastomer content to over 10 wt.%. It is obvious that, with greater contents of TPE in the PP, the effectiveness of chemical interaction of the components decreases, while in products of the thermomechanical degradation of free TPE, as a result of the presence of which a free rubber phase is formed, the formation of crosslinked structures raising the level of viscosity of the material is possible. Displacement of the curves of the relaxation spectrum towards greater relaxation times is connected with slowing down of the relaxation effects in the material melt (Figure 6, curve 2). In some studies [6, 7] the change in viscosity in the modifi ed thermoplastics is connected with the compatibility of the components of the forming mixture and with transition of the system from a single-phase into a two-phase state, and vice versa. The fall in viscosity is connected with microseparation of the system into two phases and the appearance of excess free volume which is localised at the phase boundary [8]. Thus, an inadequate influence of the content of introduced modifier on the rheological and relaxation properties is observed, which is due to change in the composition and quantity of the structures and degradation products formed. At the next stage, the influence of the presence of modifying additives on the dynamic mechanical properties of polypropylene and the position of the temperature regions of relaxation transitions was determined, since changes in the chemical and supermolecular structure of the modified polymer, leading to a change in the thermodynamic flexibility of the macromolecules, have a direct effect on the temperature regions of the relaxation transitions. In connection with this, at the next stage of the work, the nature of the relaxation processes in the initial and modified PP was studied by means of dynamic mechanical analysis (DMA). The temperature dependences of the mechanical loss tangent tg d and the mechanical loss modulus E of specimens of modified PP with heating at a rate of 5 K/min were obtained. The relaxation spectra of the initial and chemically modifi ed PP were taken in the temperature range from 180 to 250 C with a constant frequency of vibrations of 1 Hz. T/24 International Polymer Science and Technology, Vol. 33, No. 1, 2006

5 The relaxation transitions are dependent upon the kinetic flexibility of the polymer chains and forces of intermolecular interaction. The lower the intra- and intermolecular interaction, and the lower the chain flexibility, the less mobile are the units and the higher are the temperatures at which the maximum mechanical losses are observed, and vice versa. To trace the influence of graft forms in DST-30R-modified PP on the relaxation behaviour of specimens, relaxation spectra of the initial PP and modified specimens, and also of extracted and non-extracted specimens of chemically modified PP, were compared. On the temperature dependences of the mechanical loss tangent and mechanical loss modulus of the initial PP, maxima appear that correspond to a-, b-, and g-transitions at a temperature T 1 = 142 C, T 2 = 34 C, and T 3 = 152 C. At T 4 = 91 C, softening of the polymer and its subsequent transition into the viscous flow state begins. For modified PP the maxima of the mechanical loss tangent are displaced towards lower temperatures, which indicates an increase in the mobility of sections of the macromolecule chains in these zones. In all spectra of chemically modified PP not subjected to extraction, in the low-temperature region (60 70 C) maxima appear on the curve of the mechanical loss tangent and on the curve of the temperature dependence of the mechanical loss modulus, resulting from the presence in the PP of an elastomer phase in the free state and corresponding to its segmental mobility. The temperature of these transitions increases with increase in the amount of modifier introduced. It was also noted that, with increase in the elastomer content, there is a shift in the maxima of the a-, b-, and g-transitions towards lower temperatures, broadening of the temperature region of the transitions, and an increase in the area beneath them. This is probably connected with an increase in the defectiveness within the crystalline regions on account of an increase in the degree of branching. The same effect is observed when specimens of modified PP subjected to extraction are investigated. In the high-temperature region it is possible to observe differences in the relaxation behaviour of the systems investigated. Transition into the viscous fl ow state for modified PP occurs at lower temperatures by comparison with unmodified polymers and a polymer modified at low shear rates, where grafting is negligible. This may be connected with increase in the defectiveness of the crystallites formed during crystallisation of the branched polymers. High-temperature transition is normally associated with molecular mobility of the physical units in the amorphous phase and microblocks. The discreteness of this transition for a modified polymer indicates the different degree of density in different areas of the amorphous phase (intercrystallite, interspherulite, and so on), while the recorded absence of a multiplet nature of this transition confirms the high relaxation mobility in all zones of the amorphous phase. On the basis of an analysis of the conducted investigations, it was concluded that the modification of PP in processing is effective and leads to broadening of the range of control of the rheological and relaxation properties of material melts. Reduction in the level of viscosity during processing and acceleration of relaxation processes make it possible to carrying out processing at temperatures K lower than the processing temperature of pure PP. The formation of block and graft products of interaction of PP and modifier has been discovered and investigated. Using a mathematical model of extrusion processes [5], the optimum regime for modification of PP with elastomeric substances during processing has been determined: a shear rate in the zone of intense action of s 1 ; a residence time in the zone of intense action of 7.82 s; a shear stress in the melt of 1278 MPa; normal stresses of 38.5 MPa; a temperature of action of 190 C. A study of the effect of elastomer additives on the process of crystallisation and the morphology of the supermolecular structures of modified PP has been carried out in the next stage of the research. REFERENCES 1. P. V. Kozlov and S. P. Papkov, Physicochemical principles of plasticisation of polymers. Khimiya, Moscow, 1982, 224 pp. 2. D. V. Ivanyukov and M. L. Fridman, Polypropylene (properties and application). Khimiya, Moscow, 1974, 272 pp. 3. M. L. Kerber, Development of physicochemical principles of effective methods for producing composite materials. Doct. Chem. Sci. thesis, Moscow, 1981, 434 pp. 4. G. M. Danilova and Yu. V. Gurvich, Processing features of processes in low- and high-speed extruders. Plast. Massy, No. 11, G. M. Danilova-Volkovskaya, Control of properties of polypropylene during screw-disc extrusion. M. Tech. Sci. thesis, Moscow, 1993, 251 pp. 6. R. V. Torner and G. M. Danilova-Volkovskaya, Procedure for calculating rheological and relaxation properties of melts of polymeric materials according to capillary viscometry. Plast. Massy, No. 5, L. Yu. Ogrel et al., VINITI Dep. No. 918-V-90, submitted 15 February1990, 7 pp. 8. Yu. S. Lipatov, Interphase effects in polymers. Naukova Dumka, Kiev, 1980, p Rapra Technology Limited T/25

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