2020 Volume 3 Issue 5

INEOS OPEN, 2020, 3 (5), 182–187  Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF Electronic supplementary information DOI: 10.32931/io2025a

Effect of Temperature on the Properties of the Rolled Composites
Based on Polyethylene and Rubber Particles

Abstract

 

The stress–strain properties of the isotropic and rolled composites based on polyethylene (PE) and rubber particles are studied. The mechanical properties are determined at 20, 50, and 70 °С. It is shown that one of the factors ensuring the maintenance of plasticity of the filled rolled PE is an increase in the crack resistance of the matrix, which is manifested in the changes of the forms of defects that arise during tension. At the rubber content no more than 20 wt %, a temperature rise leads to a change in the mechanism of the composite fracture from brittle to ductile. It is shown that the positive effect of rolling of the rubber plastics is retained during their testing at elevated temperatures.

Key words: filled polymer, rolling, stress–strain properties.

 

Introduction

In recent years, the problem of utilization of polymer wastes has undergone a qualitative change to a global challenge of modern civilization. The most important part of this problem is the cost-efficient recycling of rubber products and, first of all, worn automobile tires, which serve as a source of long-term environmental pollution. One of the methods for their processing is grinding. The resulting rubber powder can be used to obtain regenerates and sorbents and can be introduced into concretes, bitumens, polishes, and pavings [1–7]. An intention to create a technology for rational utilization of ground rubber wastes promoted the development and production of composite materials based on thermoplastic polymers and elastic fillers [5–7]. These dispersion-filled composites were termed the rubber plastics [8].

A distinguishing feature of rubber particles from conventional mineral (rigid) fillers consists in the fact that Young's modulus of elastic particles is less than that of a thermoplastic matrix. As a consequence, rubber particles can be deformed along with a matrix polymer, which results in changes in the stress distribution both around the particles and in the polymer block [9–11]. This circumstance makes rubber plastics interesting research objects from the viewpoint of their mechanical properties, deformation mechanics, and fracture of structurally nonuniform systems in general. However, the combined deformation of the matrix and elastic particles requires good adhesion between the polymer and the filler. Different approaches are used to enhance the level of adhesion at the polymer–rubber particle boundary, ranging from the introduction of various compatibilizers into the composition of rubber plastics to the polymerization of the matrix polymer in the presence of the elastic filler. The results of numerous investigations in this field were summarized in several reviews [2–7, 12–14].

Another peculiarity of the ground rubber as a polymer filler is the particle sizes, which can reach hundreds of microns. The debonding or break of large particles facilitates the appearance of critical defects (cracks and rhomboid pores) which growth leads to the material fracture. Thus, studying the mechanical properties of the composites based on low-density PE, which is widely used in the production of rubber plastics, it was found that the appearance of only one defect caused by debonding (low adhesion at the matrix–filler interface) or break (high adhesion between the matrix and the filler) of rubber partciles, is enough for the fracture of the whole sample [15]. This effect of a single defect is observed also in the composites based on PP [16].

The term of a large particle was defined for the polymers that deform with the formation of a neck in Ref. [17]. The particle is assumed to be large if its diameter D exceeds the critical value D_c which is defined as follows:

(1)

Here d_с is the critical crack expansion in the initial polymer, λ_d is the polymer neck draw ratio. It should be noted that at D < D_c the deformation behavior of rubber plastics upon an increase of the filler content is defined by the properties of a matrix polymer, more specifically, by the ratio between its yield stress, neck propagation stress, and tensile strength [18, 19]. The mentioned mechanical parameters of the matrix are calculated based on the initial sample cross-section.

The value of d_с is one of the crack resistance characteristics of a polymer. According to formula (1), an increase in the value of d_с or a reduction in the value of λ_d of the matrix polymer leads to the growth of D_c and, consequently, to a reduction in the probability of the appearance of critical defects in the region of a forming neck and the material break at low strains. Indeed, it was shown earlier that an increase in d_с with a rise in the experimental temperature does not lead to critical defects in the neck region of rubber plastics and the samples can be deformed up to several hundreds of percents [20]. At 20 °С, the rubber plastics of the same compositions and particle dispersities undergo fracture during the neck formation due to the appearance and growth of rhomboid pores. In turn, a reduction in λ_d as a result of cold rolling serves as a reason for a drastic increase in their deformation ability since the critical defects fall beyond the strain range of the neck growth in the composite samples [21, 22]. The experimental results obtained allow us to assume that the methods and techniques aimed at changing the characteristics of a matrix polymer such as d_с and λ_d can become a good basis for a new strategy for controlling the mechanical properties of not only rubber plastics but also dispersion-filled composites.

The rolling of polymers and composites is a technique that allows for improving their operational characteristics [22–26]. The oriented structure of a matrix polymer is formed under the action of compression and shear. There is a question about the preservation of a positive effect of rolling on the properties of materials at elevated temperatures. An answer to this question is of high practical importance for rubber plastics which are widely used as roofing materials, soft corrugated fiber cement sheets, and roof tiles and are exploited in the summertime at elevated temperatures [2–8].

The goal of the present work is to study the effect of temperature on the mechanical properties of the preliminarily rolled composites based on polyethylene and rubber particles.

Results and discussion

PE in the isotropic state used as a matrix is deformed with the formation of a neck and breaks during its growth (Fig. 1a, curve 1'). The absence of the third stage of deformation that corresponds to deformation strengthening of a sample is a reason for embrittlement of the composites on its base. To switch from ductile to brittle break of this polymer, one or two particles are usually enough [27, 28]. At 20 and 50 °С, the isotropic composites on its base undergo brittle fracture at С_f ≤ 30 wt % (Figs. S1a,b in Electronic supplementary information (ESI)). The fracture is initiated by the cracks that arise in the vicinity of the filler particles and grow perpendicular to the tension axis (Fig. S2 in ESI). At 70 °С, the isotropic rubber plastics undergo fracture during the neck formation (Fig. S1c in ESI).

     

Figure 1. Stress–strain curves of the preliminarily rolled samples of the composite based on PE at 20 °С (а), 50 °С (b), and 70 °С (c). The filler content: 0 (1), 10 (2), 20 (3), and 30 (4) wt %. The rolling degree: 2.5. Curve 1' refers to isotropic PE.

Figure 1 shows the typical stress–strain curves of the rolled samples of PE and the composites on its base tested at different temperatures. The rolling leads to a qualitative change in the form of the stress–strain curves of both neat PE and the rubber plastics. At 20 °С, the rolled polymer is deformed with the formation of a neck, it undergoes fracture at the stage of deformation strengthening (Fig. 1а, curve 1). An analogous deformation behavior was observed for the samples of the rolled composite bearing 10 wt % of the rubber (Fig. 1а, curve 2). They also break at the third stage of tension. Consequently, the rolling of the composites with С_f ≤ 10 wt % hampers their brittle fracture, and they retain the plastic properties. The reasons for this effect are not only stable growth of the matrix polymer neck (the presence of deformation strengthening during uniaxial tension) and a reduction in the neck draw ratio [22] but also the variation of defects that result during tension. The uniaxial tension of the preliminarily rolled rubber plastic leads to the formation of oval pores in the region of a neck that are caused by the fracture of the filler or debonding of the filler particles from the matrix (Fig. 2а). The oval-shaped pores grow only along the tension direction and are not critical for the material due to the absence of transverse expansion. As the filler content increases to 20 wt %, the deformation behavior of the rolled composites changes. Their elongation is accompanied by the formation of a neck but the break occurs at the initial stage of its growth along the working part of the sample (Fig. 1а, curve 3). According to the results of microscopic studies, in this case the sample fracture is caused by the appearance and growth of rhomboid pores in the region of the forming neck (Fig. 2b). Their expansion across the sample cross-section leads to the material fracture [17, 19]. Along with the critical defects, the oval pores are also formed in the vicinity of the filler particles. As the filler content increases further to 30 wt %, the rolled rubber plastics undergo fracture during uniaxial tension at the stage of the neck formation (Fig. 1а, curve 4).

Figure 2. Defects that are formed during tension of the rolled composites: oval pores in the neck region of the composite PE–10 wt % of the rubber particles; the test temperature 20 °С (а); rhomboid pore formed in the composite PE–20 wt % of the filler before fracture; the test temperature 50 °С (b); split pores in the neck region of the sample PE–20 wt % of the filler; the test temperature 70 °С (c).

Based on the results presented, it can be concluded that one of the factors for maintenance of plasticity by the filled polymers is an increase in the crack resistance of the matrix. An increase in the crack resistance of PE as a result of rolling is manifested in the variation of defects that are formed in the composites on its base. Depending on the filler content in samples, there are formed oval or rhomboid pores. As a consequence, the ductile fracture is accomplished at the stage of deformation strengthening (С_f ≤ 10 wt %) or during neck propagation (С_f ≥ 20 wt %).

The absence of transverse cracks upon tension of the rolled composites at С_f ≤ 10 wt % is likely to be connected with the effect close to that of suppression of craze formation as a result of orientation of glassy polymers, namely, the growth of the preliminary orientation degree leads to a more rapid increase in the stress of crazing in a glassy polymer compared to the growth of the material yield stress [29, 30]. As a result, at a certain degree of preliminary orientation of a glassy polymer, its ability to undergo deformation by a crazing mechanism can be fully lost. As a rule, in this case, there is observed a drastic increase in the plasticity of a glassy polymer.

At 20 °С, the positive effect of the preliminary rolling is retained only for low-filled rubber plastics (С_f ≤ 10 wt %). As it was noted above, at С_f ≥ 20 wt %, the tension of the preliminarily rolled composite results in critical rhomboid defects. Obviously, variation of the geometry of oval pore leaflets from rounded to wedge-shaped and its further growth perpendicular to the tension axis is caused by an increase in the level of local stresses at the particle equator due to overlapping of the stress fields of adjacent particles. The overlapping of the stress fields is evidenced also by the fact that the thickness of a layer between particles at С_f  ≥ 20 wt % becomes lower than their diameter. Thus, a ratio of the thickness of a polymer layer L to the diameter of a particle D can be evaluated using the following equation [31]:

(2)

where V_f is the volume fraction of the filler. At V_f ≥ 17 vol % (С_f ≥ 20 wt %), the ratio L/D ≤ 0.55 and satisfies the condition of overlapping of the stress fields around adjacent particles during sample tension [9–11, 31].

The fact of the appearance of rhomboid pores in the composites since some concentration testifies that the critical pore expansion, i.e., its size at which an innocuous defect converts to a critical one, depends on the filler concentration. Consequently, the conditions for equation (1) are necessary but not sufficient for the formation of rhomboid pores in the filled plastic polymers. While predicting the deformation properties of dispersion-filled composites, one should take into account that the value of d_с explicitly depends on the polymer filling degree.

Taking into account the fact that the reason for formation of rhomboid defects is the level of overstresses at the equator of elastic particles, it can be assumed that, upon a rise in the temperature that facilitates a general reduction in the sample stresses, the rhomboid pores at the same particle concentration will be missing. In other words, critical defects are likely to form with the higher probability at elevated temperatures than at 20 °С at the same filler concentration.

Indeed, an increase in the test temperature to 50 °С favors the retention of plastic behavior of the rolled composites in a wider range of the filler concentrations. As can be seen from curve 3 in Fig. 1b, at this temperature the deformation behavior of the rolled PE with С_f = 20 wt % changes. Its tension is accompanied by the formation of a neck, and the break occurs at the stage of uniaxial tension after its propagation. In the case of the composite bearing 30 wt % of the rubber particles, a rise in the test temperature does not change its deformation behavior. As well as at 20 °С, it breaks during the formation of a neck (Fig. 1b, curve 4).

Of note is a change in the form of a stress–strain curve of the rolled PE tested at 50 °С (Fig. 1b, curve 1). PE is deformed with the formation and growth of a neck, its break occurs during orientation strengthening. Unlike the curve of the rolled PE obtained at 20 °С, the third region of the stress–strain curve is divided into two parts: at the beginning, the stress increases with the growth of the draw ratio, and, after reaching ~600% strain, its value remains constant up to the sample fracture. The nominal breaking strength of the rolled polymer exceeds its yield stress as well as at the lower test temperatures.

At 70 °С, the preliminarily rolled polymer is also deformed with the formation and propagation of a neck (Fig. 1c, curve 1) but the curve region that corresponds to deformation strengthening changes again. After reaching the strains of about 500–600%, the stress reduces with a strain decrease. As a consequence, the third region of the polymer stress–strain curve contains a broadened maximum. At the sample break, the nominal stress of the polymer becomes lower than the neck propagation stress. In this situation, one could expect that the deformation behavior of the rolled composites will be close to that of isotropic rubber plastics, namely, the filled PE will become brittle at the low filler content. As can be seen from Fig. 1c, at 70 °С the rolled composites retain the plasticity up to the filler content of 20 wt %. Their break occurs during neck propagation. The fracture of the sample with C_f = 30 wt %, as well as that at the lower test temperatures, occurs during the formation of a neck (Fig. 1c, curve 4).

Taking into account that degeneration of deformation strengthening of PE starts from the strains above 500–600% and the nominal stress in the polymer at these strains is lower than the yield stress, one can estimate the filler concentration during a brittle-to-ductile transition in the composites based on this polymer [19]:

,

where

,

(3)

Here σ_m, σ_dm are the neck draw strength and stress for the matrix polymer. According to the calculation, V* = 0.4 vol %. The resulting value of V* is significantly lower than the experimental one: V = 17 vol %. This implies that the preliminarily rolled composite does not follow general regularities established for isotropic dispersion-filled composites.

Figure 2c shows the image of a surface of the rolled sample of the composite PE–20 wt % of the rubber particles fractured at 70 °С. There is a clearly defined structure of the matrix polymer. The rubber particles are deformed and debonded from the matrix. The resulting defects that have the form of split pores are not propagated across the sample and are not critical. The oriented matrix polymer hampers their transverse growth.

Hence, the deformation behavior and fracture character of the rolled rubber plastics are stipulated, first of all, by the type of defects that arise in the material during mechanical tests. The ratios between the strength characteristics of the matrix polymer such as yield stress, neck draw stress, and strength are not the factors that dictate the deformation behavior of the preliminarily rolled rubber plastics at elevated temperatures. The main condition for maintaining the plasticity of the rolled rubber plastics includes the ability of the oriented matrix polymer to withstand the formation of critical defects, i.e., the crack resistance of the matrix polymer and, as a consequence, the shapes of defects formed during mechanical tests. At elevated temperatures, a reduction in the level of stresses in the composite samples upon uniaxial tension allows one to decrease the probability of the appearance of critical defects and facilitates the retention of plasticity by the rolled samples.

Figure 3 depicts the diagrams of changes in the yield stress and strength of the rolled composites at different temperatures. Here one can see the known effects of a reduction of these characteristics with the growth of the elastic filler content and an increase in the sample test temperatures.

   

Figure 3. Yield stress (а) and tensile strength (b) of the preliminarily rolled composites (λ_r = 2.5) depending on the filler content. The test temperatures were as follows: 20 (1), 50 (2), and 70 °С (3).

Figure 4 shows the dependences of the relative elongation at break (ε) for the rolled rubber plastics bearing different contents of the rubber particles tested at elevated temperatures. For comparison, the results obtained during tension of isotropic rubber plastics are presented. The unrolled composites undergo brittle (at 20 and 50 °С) or quasibrittle (at 70 °С) fracture at the low strain values. The preliminary rolling leads to the growth of a deforming ability of rubber plastics. The observed change in deformation properties of the rolled materials is associated with a change in the character of their tension. Thus, at 20 °С the rolled composites with С_f = 10 wt %, unlike the samples of the isotropic composites, retain the plastic properties, which leads to a drastic increase in the values of ε to 460%. As the filler concentration increases, the preliminarily rolled rubber plastics undergo fracture during neck formation. As a consequence, their relative elongation at break becomes lower. At 50 and 70 °С, the values of ε reduce monotonously at the particle contents ranging from 0 to 20 wt %. The rubber plastics with the concentration of the rubber particles of С_f = 30 wt % break during the formation of a neck, independent from the test temperature. The relative elongations at break are lower than those for the less filled samples. At С_f  ≥ 20 wt % and a similar character of tension of the rubber plastics, the values of ε reduces with a rise in the test temperature.

Figure 4. Fracture strain of isotropic (a) and preliminarily rolled composites (b), depending on the filler content. The test temperature: 20 (1), 50 (2), and 70 °С (3). The preliminary rolling degree: 2.5.

Experimental

The work was performed with high-density polyethylene (PE) of 277-73 grade (OAO Kazanorgsintez). The filler was rubber particles with the sizes ranging from 10 to 800 μm, obtained by elastic-deformation grinding of the worn automobile sealers based on ethylene propylene diene rubber.

PE was blended with the rubber particles in a single-screw laboratory extruder that has two zones of heating and blending chamber. The ratio of the screw length to its diameter was equal to 12. The blending chamber consisted of coaxial corrugated cylinders with the lengths of 120 mm and the gap between them of 1 mm, the internal cylinder was a continuation of a rotor. The temperatures in the mixing zones were 160 and 170 °С. The filler concentration (С_f) composed 10, 20, and 30 wt %, which corresponds to 8, 17, and 26 vol %.

The blends obtained at 160 °С were pressed into plates with the thickness of 800 μm. After seasoning of the material under the pressure for 10 min, the temperature was gradually reduced to 20 °С. The reference sample was PE processed under the same conditions.

The resulting plates were rolled at room temperature on a laboratory unit between two rollers rotating with the same rate (cold rolling). The degree of material deformation during rolling (rolling degree λ_r) was estimated as the ratio of the intiial thickness to the thickness of the rolled plate. The rolling degree of the initial PE and the composites on its base was constant and composed 2.5. At this value of λ_r, the sample length in the rolling direction increased and, consequently, its thickness reduced, whereas their width remained almost unchanged.

The rolled plates were cut into samples along the rolling direction in the form of double-sided blades with the working areas of 6 × 20 mm. The samples of the same form were also obtained from the nonoriented plates. The mechanical tests of the materials were performed on an Instron 1122 verstaile testing machine equipped with a thermal chamber. The test tempeartures varied from 20 to 70 °С. The accuracy of the temperature measurement was ±1 °С. Prior to tension, a sample was placed into clips of the test machine and seasoned at the given temperature for 20 min at the constant strain. The same annealing allowed us to prevent an inevitable sample shrinkage during its heating. The tension rate was 10 mm/min.

The microscopic studies were carried out on a Hitachi S-520 transmission electron microscope and a DMW143 digital stereoscopic microscope.

The work was carried out using the nominal values of stresses, i.e., calculated relative to the initial cross-section of the sample.

Conclusions

The positive effect of rolling is retained during testing of the rubber plastics at elevated temperatures. The preliminary rolling enhances the crack resistance of the matrix polymer, which is manifested in the variation of defects that arise in the material during tension. At the elevated test temperatures, the rubber plastics with the filler contents no more than 20 wt % maintain their plastic properties. The elevated test temperatures of the preliminarily rolled composites facilitate stronger effect of orientation of the matrix polymer and an increase in the contribution of its crack resistance into the formation of the material properties.

Acknowledgements

This work was supported by the Ministry of Science and Higher Education of the Russian Federation (agreement no. 075-15-2020-794). The mechanical properties were measured using the equipment of the Center for Molecular Composition Studies of INEOS RAS supported by the Ministry of Science and Higher Education of the Russian Federation.

Electronic supplementary information

Electronic supplementary information (ESI) available online. For ESI, see DOI: 10.32931/io2025a

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