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ISSN 2522-1841 (Online) AZERBAIJAN CHEMICAL JOURNAL № 1 2021 ISSN 0005-2531 (Print)
UDC 678.01:532.135
RHEOLOGICAL PROPERTIES OF MIXTURES OF RANDOM POLYPROPYLENE WITH BUTADIENE-NITRILE RUBBER AND VULCANIZATES BASED ON THEM
A.J.Quliyev, N.T.Kakhramanov, *N.S.Koseva, N.B.Arzumanova
Institute of Polymer Materials, NAS of the Azerbaijan * Institute of Polymers, Sofia, Bulgaria
najaf1946@rambler. ru
Received 21.07.2020 Accepted 30.10.2020
The paper presents the resuits of studying the effects of butadiene-nitrite rubber concentration and temperature on the flow curves of the random polypropylene-based polymer blends. To improve the technological compatibihty of poiymer Mends, a graft(ed) copoiymer of poiypropyiene with maieic anhydride was used as a compatibiiizer. Meit viscosity as a function of temperature and shear rate is shown, and the activation energy of a viscous flow has been defined. A generaiized temperature-invariant characteristic of the viscosity properties of poiymer Mends has been piotted. This makes it possibie to predict the vaiue of their effective viscosity at high shear rates, ciose to reai processing conditions.
Keywords: butadiene-nitrile rubber, compatibility, melt viscosity, rheology, activation energy, compa-tibilizer.
doi.org/10.32737/0005-2531-2021-1-23-29
Introduction
In recent years, the attention of many scientists around the worid has been paid to studying the structure and properties of incompatibie poiymer composites based on poiyoiefins and poiar eiastomers. The interest to such materiais is primaiy the fact that they enabie to formation of properties in the composites compieteiy different in nature of their poiymers [1-3]. These composites inciude biends based on thermo-piastic poiyoiefins and poiar eiastomers - buta-diene-nitriie, butadiene-styrene rubbers, etc. But, aiong with this, there are probiems reiated to their miscibiiity and technoiogicai compatibii-ity. The iack of the required technoiogicai com-patibiiity of the biend components causes the stratification of biends during biending. This phenomenon naturaiiy affects the basic physicai and mechanicai properties of the biends [4, 5]. To achieve the required ievei of technoiogicai compatibiiity in poiymer biends, the most effective is the use of smaii amounts of com patibiiizers, which are, as a ruie, graft copoiy-mers. The iatter circumstance enabies not oniy to achieve the required intermiscibiiity and technoiogicai compatibiiity, but aiso to signi-ficantiy improve the rheoiogicai properties of
poiymer biends. At working out poiymer biends based on a thermopiastic poiyoiefin and a poiar eiastomer, the other important probiems reiated to production of thermopiastic eiastomers are soived. These eiastostomers not oniy have eiastic properties, they are aiso processed as thermos-piastics on equipment by such methods as extrusion and injection mouiding [6-9].
Despite the importance of the probiem under consideration, the references provide very iimited information on the rheoiogicai properties of thermopiastic eiastomers and vuicani-zates based on them. Therefore, in this paper, the main attention is paid to studying the rheoiogicai features of the fiow of thermopiastic eiastomers and vuicanizates based on them.
Experimental part
Random polypropylene (RPP), or a thermopiastic statisticai ethyiene-RP2400 with propyiene copoiymer is characterized by the foi-iowing properties: uitimate tensiie stress - 28.5 MPa, tensiie strain - 600%, MFI - 1.78 g/10min, density - 0.904 g/cm3, meiting point - 1460C, heat Vicat resistance according to - 1310C.
SKN is a butadiene-nitriie rubber containing a 40% acryionitriie (SKN-40).
Exxelor PO1200 compatibilizer is intended for use as a binding agent in order to improve the compatibility of polymeric materials and the reactivity under interacting with inorganic fillers.
This is achieved in a reactive extrusion process by the graft copolymerization of polypropylene containing maleic anhydride (MA). The MA concentration in the compatibilizer is 3.0 mass %.
Polymer composites based on RPP+SKN were obtained in the process of blending on rollers at 1700C. After the melting of RPP, 2.0 mass% PMPA was added, and then, in the process of intensive mixing SKN crumbs were added on rollers for 8 min. To obtain vul-canizates 2.0 mass% sulfur was additionally added during blending.
Rheological studies of polymeric materials were carried out using INSTRON (Italy) rheome-ter, a capillary rheometer, MELT FLOW TESTER, CEAST MF50 within 170-2300C temperature range and under 2.16-21.6 kg loads.
Results and its discussion
It is quite obvious that the lack of systematic studies on the rheology of polymer blends, in a number of cases, does not allow to give theoretical validation of those processes that occur during melting. At as a minimum, these are three-component blends consisting of a thermoplastic, an elastomer and a compatibi-lizer. The use of a low molecular weight grafted copolymer based on polypropylene and maleic anhydride (PP-g-MA) as a compatibilizer promotes an increase in the adhesive contact between the dispersed phase of SKN and the dispersed medium of RPP, that ultimately significantly improves the technological compatibility of the mixed components of the mixture. In a viscous-flow state, the main purpose of PP-g-MA is the manifestation of plasticizing properties improve the rheological properties of polymer blends. This is due to the fact that PP-g-MA is a low molecular weight polymer with MFI=36 g/10 min, allows it to exhibit the properties of an internal lubricant agent.
There is reason to believe that the polar MA units will be located in the SCN boundary phase, and the polypropylene segments, that do
not contain grafted units, will be distributed in the RPP dispersed medium. In this case, the purpose of the compatibilizer, as a low molecular weight PP-g-MA, is not only to increase the adhesive contact in the interface region, but also to plasticization of the composite material [10]. In this case, the blends based on RPP+40 mass% SKN-40 exhibit the properties of dynamically vulcanized thermo elastoplasts (DVEP). As a lubricant agent, PP-g-MA helps maintain the melt fluidity of polymer blends and DVEP at a level acceptable for processing. So, for example, as a result of experimental studies of the MFI polymer blends, it was found that the MFI of a compatibilized polymer blend of RPP+40 mass% SKN-40 after sulfuric vulcanization was 0.7-1.2 g/10 min, while for the unmodified composition the value of this indicator was very low, i.e. within 0.1-0.3 g/10 min.
Figure 1(a, b) shows microphotographs of the polymer blend before (Figure 1 a) and after (Figure 1 b) vulcanization. From Figure 1 a, in the presence of PP-g-MA, the finely dispersed elastomeric phase is located in the form of islands in a RPP dispersed medium. Such supramolecular structure explains the increase in the elasticity of the polymer blend responsible for the deformation properties of the composite material [10-12]. It should be noted that in the process of sulfur vulcanization, the cross-linking occurs at the site of the breaking of the double bonds contained in the SKN macromol-ecule. Selective crosslinking takes place for the simple reason that in comparison with SKN, RPP macrochains do not have vinyl groups [12]. In addition, sulfur is an 8-membered cycle which, under the action of shear stresses and temperature, decomposes and is followed by the formation of radicals that initiate breaking and crosslinking at double bonds. Taking into consideration the selective nature of the crosslinking process, a DVEP based on RPP+SKN-40 has been obtained as a result of mechanochemi-cal synthesis. Figure 1b shows a microphotograph for the vulcanizate of this blend. Comparing these images, it can be seen that, as a result of sulfur vulcanization, crosslinked elastomeric formations behave as fine fillers in the RPP
composition. The peculiarity of sulfuric vulcanization is manifested in the fact that crosslink-ing selectively occurs mainly at the site of breaking the SKN double bonds. In other words, the crosslinked dispersed phase of SKN in a RPP dispersed medium behaves like an elastic filler. Such a structural organization is a DVEP characteristics that significantly affects the regularity of changes in rheological characteristics, especially of sulfur vulcanizates RPP+SKN.
Fig. 1. Microphotographs of compatibi-lized polymer mixture RPP+40%mass. SKN-40 (a) and sulfuric vulcanizaites based on them (b).
For a comprehensive analysis of the rheological properties of the considered polymer mixtures, Figure 2 shows the flow curves of RPP+SKN-40 polymer mixtures with different contents of the elastomeric component. From a comparative analysis of the flow curves in this figure, it can be seen that at low shear
stresses, the largest Newtonian flow region is not fixed. Moreover, with an increase in the concentration of SKN-40 in the composition of the RPP, a monotonic decrease in the shear rate is observed. It is characteristic that the flow curves of polymer mixtures are located between the flow curves of the initial RPP (Figure 2,1) and the initial SKN-40 (Figure 2,5). Such a uniform change in the flow pattern of polymer mixtures depending on the shear stress and concentration of SKN-40 indicates good compatibility of the components of the mixture in the presence of PP-g-MA.
Fig. 2. Flow curves of the RPP+SKN-40+PP-g-MA (1-5) polymer blends and sulfur vulcanizates based on them (2'-4') with different content of elastomer, mass%: 1(o) -initial RPP; 2,2' - 20; 3,3' - 30; 4,4' - 40; 5 -initial SKN-40.
For a comprehensive analysis of the rheological properties of the considered polymer blends, Figure 2 shows the flow curves of RPP+SKN-40 polymer blends with different content of the elastomeric component. From a comparative analysis of the flow curves given in this figure, it is seen that at low shear stresses, the largest Newtonian flow region is not fixed. At the same time, with an increase in the concentration of SKN-40 in the RPP composi-
tion, a monotonic decrease in the shear rate is observed. It is characteristic that the flow curves of polymer blends are between the flow curves of the initial RPP (Figure 2,1) and the initial SKN-40 (Figure 2,5) polymers. Such a uniform change in the flow pattern of polymer blends in dependence in the shear stress and the SKN-40 concentration indicates good compatibility of the blend components in the presence of PP-g-MA.
The same figure shows the flow curves of sulfur vulcanizates of polymer blends (Figure 2, curves 2'-4'). As expected, vulcanizates are characterized by lower shear rates. With this, the higher the content of the elastomeric component in the vulcanizate, the significantly lower the shear rate becomes. Such a high sensitivity of the shear rate of vulcanized samples to the concentration of the elastomeric component and the shear stress is primarily connected with a special nature of the formation of flexible structural units, or associates, in the melt. The difference is in the fact that the mechanism of the emergence and decay of associates in vulcanized systems is fundamentally distinguish out from the flow mechanism of the conventional homopolymers. This is due to the fact that in the process of sulfur vulcanization, crosslinked elastomeric associates are formed which, apparently, differ separately in resistance to thermal fluctuation decomposition and behave like the filler particles. However, we assume that cross-linked elastomeric associates in the melt, due to polar nitrile groups and adhesive forces can be combined into "macroassociates" which, like associates formed from the RPP macrochains, can undergo thermal fluctuation decay and recovery. In this connection, we believe that it definitely results from exposure to high shear stresses whereafter macroassociates decompose into individual associates, thereby contributing to a noticeable increase in the shear rate. For this, with an increase in shear stress, an almost linear increase in the shear rate of vulcanizates containing 20-30 mass % SKN-40 occurs. For the SKN-40 concentration equal to 40 mass %,
delinearization is observed. This phenomenon can be probably interpreted by the formation, as mentioned above, of the spontaneous thermofluctuation decay of macroassociates.
Figure 3 shows the flow curves of the RPP+40 mass% SKN-40+2.0 mass % PP-g-MA polymer blend within the 170-2300C tempera-
3.5 4 4.5 5 lgT
Fig. 3. Flow curves of the polymer mixture RPP+SKN-40+PP-g-MA at various temperatures, 0C: 1 - 170, 2 - 190, 3 - 210, 4 -2300C.
ture range. Analyzing the curves in this figure, you can see that for a constant shear stress (lgT=4.5) with increasing the temperature and shear stress, an increase in the shear rate of the polymer blend melt is observed. From this figure, it can be established that, regardless of the temperature and shear stress, the Newtonian flow region is not observed. In addition, if we pay attention to the flow curve of the composition at 1700C (Figure 3.1), we can see that in the entire range of shear stresses at this temperature, the shear rate is noticeably lower. This circumstance indicates that at this temperature the melt viscosity increases so much that certain difficulties arise with the movement of the associates in macrochains during the process of uniaxial deformation in a capillary.
For more information on the flow nature of polymer blends, on the Figure 4 shows the
■gil
3 -I-,-,-,-,-
-0.5 0 0.5 1 1.5 lgy
Fig. 4. Dependence of the viscosity of the polymer mixture RPP+SKN-40+PP-g-MA on the shear rate at various temperatures: 1 - 170, 2 - 190, 3 - 210, 4 - 2300C.
Igil
3 H-1-1-1-1-
1.9 2 2.1 2.2 2.3 1000/T
Fig. 5. Dependence of viscosity on inverse temperature at various values of shear stress, lgx: 1 - 3.8, 2 - 3.9, 3 - 4.5, 4 - 4.9.
lgi}npp't)0
Fig. 6. Generalized temperature-invariant characteristic of the viscosity properties of polymer mixtures and vulcanizates based on RPP+SKN-40 mixtures.
dependence of the melt viscosity on shear rate in logarithmic coordinates in the 170-2300C temperature range. The dependency curves in this figure are mainly presented in the form of a linear relationship, that also confirms our arguments related to the uniform dispersion of the elastomeric component in the RPP composition. In the contrary case, nonuniform and nonlinear dependences of the melt viscosity on the shear rate were observed.
Another important rheological characteristic of polymers is the activation energy of viscous flow, which is determined from the dependence of the melt viscosity on the inverse temperature (Figure 5). The activation energy of viscous flow determined from the Frenkel-Eyring equation characterizes the structural state of the associates in macrochains during its
uniaxial deformation. According to this theory, the movement of molecules in liquid is mainly determined by two factors: the first one is volume-free macrochains in the volume of the liquid given, and the latter is overcoming the forces of intermolecular interaction [13-15]. From the analysis results obtained, it follows that the apparent activation energy of viscous flow in the temperature range of 210-2300C is 51 kJ/mol, while in the temperature range of 190-2100C this value varies within 22-24 kJ/mol, and is 49-68 kJ/mol at 170-1900C.
Numerous experiments on polymer rhe-ology, carried out under the guidance of professors G.V.Vinogradov and A.Ya.Malkin, have shown that the change in the viscosity of various polymer melts under the influence of deformation is convenient to be compared when
assessing the ratio of the apparent viscosity (napp) to its maximum value (по). The ratio napp/n0 qualitatively characterizes the deviation degree of the viscosity of the polymer melt from the highest Newtonian viscosity [13-15].
Figure 6 shows the temperature-invariant characteristic of the viscosity properties of thermoplastic elastomers based on RPP+SKN-40. Analysing this figure, we can established that the form of this function does not depend on temperature, and therefore, it is called invariant in relation to temperature.
As you can see from the figure, with an increase in the reduced shear rate у-^о, a monotonic decrease in the reduced viscosity napp/n0 is observed. The existence of a temperature-invariant characteristic of the melt of polymer blends considered indicates a qualitatively identical nature of the destruction of associates in a viscous-fluid state.
Conclusion
Thus, on the basis of the above-mentioned, we can conclude that the regularity of changes in the flow curves and viscosity of the melt of RPP+SKN polymer blends and their vul-canizates, depending on the shear stress, temperature and the ratio of the mixture components, has been investigated. It is shown that, as a result of sulfur vulcanization of the RPP+SKN-40+PP-g-MA mixture, it is possible to obtain DVEP, the rheological properties of which make it possible to process them using standard equipment by injection moulding and extrusion methods. The results of a theoretical analysis of the flow mechanism of RPP+SKN polymer blends and vulcanizates based on them are presented. The activation energy of the viscous flow of the initial RPP+SKN polymer blends has been determined. A temperature-invariant characteristic which allows predicting the viscosity properties of polymer blends in relation to the real conditions of their processing by injection moulding and extrusion has been plotted.
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RANDOM POLiPROPiLENiN BUTADiEN-NiTRiL KAUCUKU iLO QARI§IQLARININ VO ONLAR OSASINDA VULKANiZATLARIN REOLOJi XASSOLORi
A.C.Quliyev, N.T.Qahramanov, N.S.Koseva, N.B.Arzumanova
i§da butadien-nitril kaugukunun miqdannin va temperaturun random polipropilen asasinda polimer qan§iqlarinin axma ayrilarina tasirinin tadqiqinin naticalari verilmi§dir Polimer qan§iqlannm texnoloji uygunlugunun yax§ila§dinlmasi ügün kompatibilizator kimi polipropilenin malein anhidridi ila tikilmi§ birgapolimerindan istifada olunmu§dur. Orintinin özlülüyünün temperaturdan va yerdayi§ma süratindan asililigi göstarilmi§, özlü axmanin aktivla§ma enerjisi müayyan olunmu§dur. Real emal §araitina yaxin yüksak yerdayi§ma süratlarinda polimer qan§iqlarinin effektiv özülüyünün qiymatini proqnozla§dirmaga imkan veran polimer qari§iqlarin özlülük xassalarinin ümumila§dirilmi§ temperatur-invariant xüsusiyyati qurulmu§dur.
Agar sözlzr: butadien-nitril kauguku, uygunluq, arintinin özlülüyü, reologiya, aktivh§m3 enerjisi, kompatibilizator.
РЕОЛОГИЧЕСКИЕ СВОЙСТВА СМЕСЕЙ РАНДОМ ПОЛИПРОПИЛЕНА С БУТАДИЕН-НИТРИЛЬНЫМ КАУЧУКОМ И ВУЛКАНИЗАТОВ НА ИХ ОСНОВЕ
А.Д.Гулиев, Н.Т.Кахраманов, Н.С.Косева, Н.Б.Арзуманова
В статье приводятся результаты исследования влияния концентрации бутадиен-нитрильного каучука и температуры на кривые течения полимерных смесей на основе рандом полипропилена. Для улучшения технологической совместимости полимерных смесей в качестве компатибилизатора использовали привитой сополимер полипропилена с малеиновым ангидридом. Показана зависимость вязкости расплава от температуры и скорости сдвига, определена энергия активации вязкого течения. Построена обобщенная температурно-инвариантная характеристика вязкостных свойств полимерных смесей, позволяющая прогнозировать значение их эффективной вязкости при высоких скоростях сдвига, близких к реальным условиям переработки.
Ключевые слова: бутадиен-нитрильный каучук, совместимость, вязкость расплава, реология, энергия активации, компатибилизатор.
