Effect of temperature on the properties of rubber-filled plastics based on medium-density polyethylene

Transcription

1 Plasticheskie Massy, No. 7, 04, pp Effect of temperature on the properties of rubber-filled plastics based on medium-density polyethylene O. A. Serenko, I. N. Nasrullaev, G. P. Goncharuk, and S. L. Bazhenov N. S. Enikolopov Institute of Synthetic Polymeric Materials of the Russian Academy of Sciences and the Moscow Pedagogical State University Selected from International Polymer Science and Technology, 31, No. 10, 04, reference PM 04/07/06; transl. serial no Translation submitted by P. Curtis The introduction of elastic or rigid filler particles into a thermoplastic polymer leads to a reduction in the deformability of the material [1]. At a certain critical degree of filling, the limiting elongation of the composite decreases sharply, at least by two orders of magnitude, and does not exceed 10 % [2]. The material becomes brittle. The filler concentration at which embrittlement of the composite occurs is dependent upon the the degree of strain hardening of the matrix polymer (the ratio of tensile strength to lower yield point) and is normally no higher than vol.% [2, 3]. According to [2 4], embrittlement of filled thermoplastics is due to the formation of a neck in the matrix polymer. At a critical filler content, the neck ceases to propagate into the working part of the specimen, and the material fails as the neck is formed. The magnitude of the limiting elongation of the composite with this type of failure is small. If a matrix polymer, for example, an ethylene vinyl acetate copolymer [5], polytetrafluoroethylene [2], or a rubber [1], is deformed without the formation of a neck, then composites based on them are not embrittled, and the limiting elongation of such materials decreases monotonically with increase in the degree of filling. There have been very few studies of the effect of increased temperatures either on the mechanical characteristics of a composite or on the critical degree of filling at which embrittlement of a filled thermoplastic occurs. The aim of the present work is to investigate the temperature properties and mechanisms of failure of medium-density polyethylene (MDPE) filled with rubber particles. To prepare the composite material, MDPE of grade Lukoten F 3802 B and polydispersed rubber crumbs produced by the elastic deformation comminution of worn automotive tyres were used. This filler is characterised by poor adhesion to PE [6]. The particle size of the rubber was μm. The mixing conditions of the PE and elastic filler, and also the moulding conditions of sheets of the composite obtained, are given in [6]. The filler concentration was varied from 6.6 to 76 vol.% (7 80 wt.%). Specimens in the form of dumb-bell testpieces with a working part measuring 5 35 mm were cut from the sheets obtained. Mechanical tests of PE filled with rubber particles (rubber-filled plastics) were carried out on a 38R-005 dynamometric unit at an elongation rate of mm/min and temperatures of 80 C. Figure 1 presents the elongation curve of PE at room temperature. The polymer is deformed with the formation of a neck, which propagates to the entire working part of the specimen. Rupture of the PE occurs at the stage of orientation hardening. The coeffi cient of hardening of the polymer is fairly high and equal to 1.7. At elevated temperatures the nature of elongation of the PE remains the same as at room temperature, but the deformation and strength properties of the polymer change. Figure 2 gives the temperature dependences of the lower and upper yield points, dm and ym, and also the strength, m, of PE. The magnitudes of dm, ym, and m decrease linearly with increasing temperature, but the rates of reduction in the strength parameters of the polymer are different. The slope of the lines decreases in the T/12 International Polymer Science and Technology, Vol. 32, No. 1, 05

2 Figure 1. Elongation curve of MDPE at temperature of C Figure 2. Temperature dependences of strength (1), upper yield point (2), and lower yield point (3) of MDPE Figure 3. Temperature dependence of tensile strain of MDPE following order: strength yield point lower yield point. The stress of neck propagation is least sensitive to change in temperature. Figure 3 gives the temperature dependence of the breaking elongation of PE. With increase in the test temperature, the deformability of the polymer increases. The introduction of rubber particles into the polymer alters its mechanical properties, mechanism of deformation, and behaviour at elevated temperatures. Figure 4 gives the elongation curves of composites containing 7, 15, and 40 wt.% rubber particles. At room temperature, the mechanisms of failure of these materials are different. Thus, PE with 7 wt.% elastic filler is deformed by neck propagation. With a rubber content of 15 wt.%, the composite fails upon the formation of a neck (quasibrittle failure), and with a rubber content of 40 wt.% it is deformed macrohomogeneously. As can be seen from Figure 4a, increase in the temperature alters the nature of elongation of the material with 7 wt.% filler. At a temperature of 60 C, failure of the filled polymer occurs at the hardening stage. Change in the mechanism of deformation with increasing temperature also occurs in PE with 15 wt.% rubber particles (Figure 4b). At 60 C, rupture of the material occurs upon neck propagation, and at 70 C the composite fails at the orientation hardening stage. In contrast to PE containing 7 and 15 wt.% rubber, the material with 40 wt.% filler is deformed macrohomogeneously in the entire temperature range investigated (Figure 4c). Thus, with increase in the test temperature, a change in the mechanism of deformation occurs in composites International Polymer Science and Technology, Vol. 32, No. 1, 05 T/13

3 Figure 5. Temperature dependences of upper yield point (a) and lower yield point (b) of MDPE containing 0 wt.% (1), 7 wt.% (2), and 15 wt.% filler Figure 4. Elongation curves of MDPE containing 7 wt.% (a), 15 wt.% (b), and 40 wt.% (c) rubber particles at temperatures of C (1), 60 C (2), and 70 C (3) with small degrees of filling, from rupture upon neck formation to failure upon neck propagation and then to plastic rupture at the hardening stage. Note that the failure upon neck propagation is transitional from quasibrittle to plastic behaviour of the material. Figure 5 gives the dependences of the upper yield point yc (a) and lower yield point dc (b) of rubber-filled plastics with a low degree of filling. The dependences of unfilled PE are also presented for comparison. The magnitudes of yc and dc of composites with 7 and 15 wt.% filler are lower than in the initial PE and decrease linearly with increasing temperature. However, the slope of the dependences yc T and dc T decreases with increasing degree of filling. Figure 6 gives the temperature dependences of the strength c of composites with different contents of rubber particles. If the yield point of the initial PE decreases linearly with increasing temperature, then the form of the concentration curves of the strength of rubber-filled plastics is determined by the degree of filling of the polymer. Thus, the curves c T for PE with 7 and 10 wt.% contain a maximum at a temperature of ~70 C. At filler concentrations of 30 or 40 wt.%, the strength of the material decreases monotonically as the temperature increases. The presence of maxima on the dependences c T for materials with 7 and 10 wt.% rubber is connected with change in their mechanism of deformation with increasing temperature. In the temperature range 40 C, these composites fail upon propagation or formation of a neck. At T > 50 C, T/14 International Polymer Science and Technology, Vol. 32, No. 1, 05

4 Figure 6. Temperature dependences of strength of composites with filler content of 7 wt.% (1), 10 wt.% (2), 30 wt.% (3), 40 wt.% (4), and 70 wt.% (5) Figure 7. Temperature dependences of strain of composites with filler content of 7 wt.% (1), 10 wt.% (2), 15 wt.% (3), 40 wt.% (4), and 70 wt.% (5) brittle plastic transition occurs, which is accompanied with an abnormal increase in strength. With further increase in T, the strength again decreases. Considering that at T > 50 C the mechanisms of deformation of the matrix polymer and of rubber-filled plastics with 7 and 10 wt.% filler are identical (neck propagation and subsequent hardening), the decrease in strength with increasing temperature can be attributed to a decrease in the strength of the PE. Brittle plastic transition due to increase in the test temperature also occurs in a composite containing 15 wt.% rubber particles. In this case, the change in the mechanisms of failure that occurs in the temperature range C leads to the appearance of a plateau on the dependence c T. Consequently, with increase in the content of elastic filler, degeneration of the maximum on the temperature dependence of the strength of the materials occurs. Composites with a rubber particle content of 40 or 70 wt.% are deformed macrohomogeneously in the entire temperature range investigated, and their strength decreases monotonically with increasing T. Figure 7 gives the temperature dependences of the strain at failure, ε c, of the composites. For materials with a particle concentration of 7, 10, and 15 wt.%, the ε c T curves have the form of a step. The sharp increase in the deformability of these composites at T > 50 C is due to transition from quasibrittle to plastic behaviour. If the rubber-filled plastic fails macrohomogeneously in the entire temperature range, its deformability increases monotonically with increasing T (curve 5). Thus, as the temperature increases, the breaking elongation of composites with a fixed degree of filling increases, and here, with certain degrees of filling, this increase has the nature of a sudden transition. Figure 8 gives characteristic concentration dependences of strength (a) and limiting strain (b) of composites at different temperatures. With increase in Figure 8. Concentration dependences of strength (a) and strain (b) of composites at temperatures of C (1), 60 C (2), and 80 C (3) International Polymer Science and Technology, Vol. 32, No. 1, 05 T/15

5 the degree of filling, the strength and deformability of the materials decrease. Increase in temperature leads to an increase in the limiting elongation and to a reduction in the strength of the filled polymer. Increase in temperature leads to a change in the form of the dependence ε c C f. At room temperature, the magnitude of ε c decreases sharply from 740 to 36% when a small amount of rubber particles (2 wt.%) is introduced. This is connected with embrittlement of the composite [7]. At a temperature of 80 C, the limiting elongation of the material decreases monotonically with increasing filler concentration. An analysis of the elongation curves of filled PE indicates that, at a temperature of 70 or 80 C, with increase in the rubber particle content there is a successive change in the mechanisms of failure from rupture at the stage of orientation hardening to failure at neck propagation and then to macrohomogeneous deformation. Such transitions are called plastic plastic. There is no stage of quasibrittle rupture of the materials. Consequently, composites are not embrittled with increase in the rubber particle content at these temperatures. The plastic behaviour of filled PE in the entire range of degrees of filling is the cause of the monotonic decrease in deformability with increasing rubber particle concentration at 70 or 80 C. The sharp reduction in the deformability (embrittlement) of PE when a small amount of rubber particles is introduced is due to the formation of pores of rhomboidal form [7]. Their rapid growth in a direction perpendicular to the axis of elongation leads to failure of the specimen at small magnitudes of elongation. The rhomboidal pores actually slit the material. Increase in temperature suppresses the development of dangerous pores in the composite [8]. The absence of rhomboidal pores in the filled PE at elevated temperatures and the high coefficient of hardening of the matrix polymer result in high deformability of the material and prevent its embrittlement with increase in the content of rubber particles [8]. It is obvious that these facts are the reason for the brittle plastic transition that is observed in the material with 7 15 wt.% rubber particles with increasing temperature. where m is the strength of the polymer and V f is the volume fraction of filler particles. According to equation (1), the dependence of the strength of the filled polymer on the particle concentration should be described by a straight line in c V f coordinates. Figure 9 shows the characteristic dependences of the strength of the rubberfilled plastics investigated at different temperatures on V f. The lines describing the experimental results have a break connected with change in the mechanism of failure of the material. At C this is transition from plastic to quasibrittle rupture, and at 80 C it is transition from plastic to macrohomogeneous deformation. The point of intersection of the lines corresponds to the concentration V f * at which transition from one type of failure to another occurs. Figure 9. Concentration dependences of strength of rubberfilled plastics at temperatures of C (1), 60 C (2), and 80 C (3) in coordinates of equation (1) ANALYSIS OF THE RESULTS Tensile Strength The dependence of the strength of the rubber-filled plastic on the degree of filling was analysed within the framework of a grid model of the material as proposed by Smith. With poor adhesion and separation of the particles, the tensile strength of the PE rubber composite is described by a two-thirds law [8, 12] 2 3 = 1 V / (1) c m f Figure 10. Correlation of lower yield point of MDPE with values of strength. Explanations in text. Figures denote temperatures: 1 C; 2 40 C; 3 50 C; 4 60 C; 5 70 C; 6 80 C T/16 International Polymer Science and Technology, Vol. 32, No. 1, 05

6 Extrapolating the dependences c V f at V f > V f * to V f = 0, we determine the stress in the matrix polymer under quasibrittle or macrohomogeneous rupture. The values obtained are similar to the values of the lower yield point of the matrix polymer at the corresponding test temperature. Figure 10 compares the values obtained by extrapolating the dependences c V f at V f > V f * with the values of the lower yield point of PE at different temperatures. The dependence is linear and has a slope close to 45, which indicates that, at quasibrittle and macrohomogeneous failure, the strength of the composite is determined not by the tensile strength of the matrix polymer but by its lower yield point. Upper Yield Point As can be seen from Figure 5a, the upper yield point of PE and rubber-filled plastics decreases linearly with increasing temperature. The slope of the lines depends on the content of rubber particles and decreases with an increasing degree of filling. Experimental data corresponding to PE are described by the straight-line equation ym = T (2) where ym is the upper yield point of the matrix polymer. In the range from to 80 C, equation (2) takes the form ym = 024. T (3) ym where ym is the upper yield point of the polymer at C. The dependence of the upper yield point of the PE rubber composite on the filler content is defined by the formula [6] 2 3 = 1 V / (4) yc ym f where m is the upper yield point of the composite. Substituting (3) into (4), after transformations we obtain: 2 3 yc = ym ( V / f ) 0. 24( 1 V / f ) ( T ) (5) The first term in equation (5) is equal to the upper yield point of the composite at a certain rubber particle content at C, yc = 2/ V T (6) yc yc f From equation (6) it follows that the temperature dependence of the upper yield point of the composite, just like the dependence of the matrix polymer, is described by a linear function, but the slope of the line will be smaller than the slope of the dependence ym T for the initial polymer. According to equation (6), the slopes of the lines for yc T for composites containing 7 wt.% (6 vol.%) or 15 wt.% (13 vol.%) rubber particles should be equal to 0. and 0.18 respectively. Experimental dependences of these composites are described by lines having slopes of 0.21 and 0.18 respectively. A good agreement is observed between calculated and experimental values. Thus, reduction in the slope of the temperature dependence of the upper yield point of the composite with increase in the degree of filling is connected with reduction in the content of matrix polymer in the composition of the material. Lower Yield Point A similar analysis will be carried out for the temperature dependences of the lower yield points of PE and rubberfilled plastics. The temperature dependence of the lower yield point of the matrix polymer dm in the range 80 C is described by the equation dm = 015. T (7) dm where dm is the lower yield point of PE at C. The lower yield point of the composite is described not by the two-thirds law but by the linear function [6] = 1 V (8) dc dm f where dc is the lower yield point of the composite. Substituting equation (7) into equation (8), after transformations we obtain = V T (9) dc dc f where dc is the lower yield point of the composite at a certain rubber particle content at C. According to equation (9), the lower yield point of the composite, as with the matrix polymer, decreases linearly with increasing temperature, but the slope of the line decreases as the filler content increases. Thus, the lines describing the experimental data for materials with 6 and 13 vol.% rubber particles have slopes of 0.13 and 0.12 respectively, and the calculated dependences have slopes of 0.14 and Satisfactory agreement is observed between the experimental and calculated values. Consequently, reduction in the slope of the dependences dc T of the composites with increase in the degree of filling is connected with reduction in the content of matrix polymer in the composition of the rubber-filled plastics. International Polymer Science and Technology, Vol. 32, No. 1, 05 T/17

7 Thus, increase in temperature leads to a change in the mechanism of deformation of rubber-filled plastics based on medium-density PE from quasibrittle rupture to plastic macrohomogeneous failure. The transition is accompanied with an increase in deformability, and in some cases with an increase in the strength of the material, in spite of the increase in temperature. The nature of failure of the rubber-filled plastics deforming macrohomogeneously does not change with increasing temperature. The strength of the rubber-filled plastics, irrespective of temperature, is described by the two-thirds law (1). However, on quasibrittle and macrohomogeneous failure of the material, its strength is determined not by the yield point of the matrix but by its lower yield point. The upper and lower yield points of rubber-filled plastics with a low degree of filling decrease linearly with increasing temperature, but the rate of decrease in these parameters of the composite is lower than for the matrix polymer, which is due to the lower PE content in the composition of the material. This work was supported financially by the Russian Foundation for Basic Research under Grant REFERENCES 1. L. E. Nil sen, Mechanical properties of polymers and polymer composites. Moscow, S. Bazhenov, Plastics additives. Chapman and Hall, London Weinheim New York Tokyo Melbourne, 1998, p O. A. Serenko et al., Vys. Soed., A44, No. 3, 02, p A. A. Berlin et al., Symposium of Scientific Proceedings on Physical Aspects of Predicting Failure and Deformation. FTI, Leningrad, S. L. Bazhenov et al., Vys. Soed., A44, No. 11, 02, p O. A. Serenko et al., Vys. Soed., A44, No. 8, 02, p O. A. Serenko et al., Vys. Soed., A45, No. 5, O. A. Serenko et al., Vys. Soed., in press. 9. T. L. Smith, Trans. Soc. Rheology, 3, 1959, p (No date given) T/18 International Polymer Science and Technology, Vol. 32, No. 1, 05