2024 Volume 7 Issues 4–5

The main industrially implemented method of recycling rubber waste is grinding. Research into the possibility of reusing rubber crumb or powder, determining the directions of their rational, long-term use have been actively conducted since the 1980s [1, 2]. Nowadays, rubber crumb is widely used in asphalt bitumen coatings [3–5], waterproofing, roofing materials, etc. [6]. However, the volumes of consumption of crushed rubber waste are inferior to the volumes of their accumulation [7]. As a result, the investigations aimed at developing materials and products that contribute to the expansion of further application scope of rubber crumb remain highly urgent. This is evidenced by the numerous publications devoted to the problem (see, for example, only some of them: [8–14]).

The materials, the main component of which is rubber crumb, reactivate the valuable properties of elastomers, while saving primary raw materials [12]. The most successful solutions in the production of popular highly filled composites containing up to 90 wt % of rubber particles include coatings for injury-safe fields of sports facilities, running and cycling tracks, and playgrounds [11, 14–16]. For their production, epoxy [17], polyester [18] or polyurethane resins [19, 20] are used as a binder. The production stages of such coatings, including mixing rubber crumb with resin, application, leveling and subsequent curing of a binder in the composition, are similar to the technological stages of construction of road surfaces with asphalt concrete pavement. However, unlike the latter, the process of constructing soft coatings is based mainly on the use of manual labor [21]. This circumstance reduces the productivity of the process. It could seem that the use of devices and mechanisms designed to perform the main technological operations during mixing and laying of asphalt concrete mixtures would solve the problem. It can be assumed that the restraining factor is the corrosion instability of asphalt pavers to the effects of resin components used for these compositions.

Rubber plastics are composites based on polyolefins and crushed waste rubber products [6, 22]. The temperature range for mixing these components when using polyethylene as a matrix polymer does not exceed the temperature range for mixing asphalt bitumen compositions, namely, 120–160 °C [6, 22, 23]. In addition, both polyolefins and rubber crumb are not aggressive substances capable of causing corrosion of machine parts used in the construction of road surfaces. These circumstances allow us to assume that highly filled rubber plastics are potentially suitable for the production of soft road surfaces. This work is devoted to studying the effect of the composition of the matrix polymer and the filler type on the mechanical properties of highly filled rubber plastics containing 80 and 90 wt % of rubber crumb.

Results and discussion

During road construction, the convenience and quality of laying a highly filled mixture with its subsequent compaction depend on the viscosity of the composition, which, in turn, is determined by the binder viscosity and, in the case of rubber plastics, by the viscosity of the matrix polymer. The technological characteristic of the viscosity of thermoplastics is the melt flow index (MFI). This parameter also allows for estimating an average molecular weight of the polymer [24], which affects such characteristics as critical crack opening, the probability of formation of dangerous defects in the filled polymer, and, in general, the crack resistance of the matrix and the composite based on it [25–28]. For example, the critical crack opening of LDPE-2 (MFI = 2 g/10 min) is 1400 μm and is greater than the corresponding value of LDPE-3 (MFI = 8 g/10 min), which is 1050 μm [29]. The effect of the polymer MFI on its crack resistance is more clearly manifested in the case of HDPE: the critical crack opening of HDPE-1 (MFI = 0.9 g/10 min) is 680 µm and that of HDPE-2 (MFI = 20 g/10 min) is 40 µm [29].

To study the effect of the viscosity of polyethylene matrices on the properties of rubber plastics, LDPEs with the following MFI values were used: 0.3 g/10 min (LDPE-1), 2 g/10 min (LDPE-2), and 7 g/10 min (LDPE-3). Figure 1 (a, c) shows the histograms of the change in tensile strength and strain at break of the rubber plastics containing 80 and 90 wt % of EPDM rubber crumb, depending on the MFI of the matrix. As the MFI of the LDPE increase from 0.3 g/10 min to 7 g/10 min, the tensile strength of these composites decreases by only 3 MPa in the case of the EPDM content of 80 or 90 wt %. The strain at break of the highly filled rubber plastics based on LDPE only slightly depends on the MFI of the matrix polymer.

Figure 1. Tensile strength (a, b) and strain (c, d) at break of rubber plastics based on LDPE (a, c) or HDPE (b, d) and EPDM particles at different MFI values of the matrix polymers.

In the case of the HDPE-based rubber plastics, on passing from the polymer with a low MFI value to the polymer with a high MFI value, both mechanical characteristics of the material reduce (tensile strength and strain at break). The difference in the values of tensile strength between the composites based on HDPE-1 and HDPE-2, containing 90 wt % of rubber particles, is no more than 1.4 MPa, while the strain at break of the same rubber plastics decreases by 74%.

The dependence of the mechanical properties of the highly filled rubber plastics on the MFI of the matrix polymer is caused by a decrease in its crack resistance with an increase in the MFI values, or more correctly, with a decrease in its molecular weight [24–26]. However, independent of the type of polyethylene in use, its MFI, the rubber plastics with 90 wt % of rubber crumb are capable of deforming by more than 200%. Thus, the relative elongation of these composites is in the range of 220–330%, and the tensile strength is 5–10 MPa. This fact indicates the applicability of these composites as external road surfaces [12–16]. In addition, these results allow for concluding that there is no need to impose requirements on the MFI value of the matrix polymer used to obtain them.

From the viewpoint of the rational use of polymer waste, it is reasonable to use not only crushed rubber waste, but also polyolefin waste for obtaining rubber plastics. Polyethylene waste after all stages of its processing and subsequent granulation most often represents a mixture of low- and high-density polyethylenes. The content of each of them in HDPE waste or LDPE waste is a poorly controlled process due to the difficult separation of these polymers. To determine the effect of the polyethylene matrix composition on the mechanical properties of rubber plastics, we studied a number of model mixtures, changing the LDPE/HDPE ratio, namely, 100/0, 30/70, 50/50, 70/30 and 0/100. To avoid heterogeneity in the distribution of one polymer in another, the method of joint elastic deformational grinding was used. Table 3 lists the mechanical properties of the samples of the LDPE/HDPE mixtures of different compositions. At a predominant content of one of the polymers (LDPE or HDPE) in the mixture, the samples deform plastically with the formation of a neck and decompose at a strain of 150% (LDPE/HDPE = 30/70) or 470% (LDPE/HDPE = 70/30). Another deformation behavior is demonstrated by the samples from the mixture with equal LDPE/HDPE contents (50/50). They are destroyed brittlely and their strain at break does not exceed 20% (Table 3).

Figure 2 shows histograms of changes in tensile strength and strain at break of the rubber plastics based on LDPE/HDPE blends with a rubber crumb content of 80 and 90 wt %. The presence of 30 wt % of HDPE in LDPE and, conversely, 30 wt % of HDPE in LDPE does not have a negative effect on the mechanical properties of the highly filled rubber plastics compared to the composites based on 100% LDPE or HDPE, respectively. Moreover, as the proportion of HDPE in the matrix increases, the deformation and strength properties of the rubber plastics increase monotonically. In other words, there is no significant difference in the strength values, and most importantly in the strain at break between the rubber plastics based on the brittle matrix of the LDPE/HDPE composition = 50/50 and based on the plastic matrices with a lower or higher content of LDPE in HDPE (30/70 or 70/30). Moreover, the rubber plastic sample based on LDPE/HDPE = 50/50 is close in properties to that based on HDPE.

The observed effect of leveling the brittle deformation behavior of the matrix when introducing 80 or 90 wt % of rubber particles into it is due to the specific mechanism of deformation of highly filled rubber plastics [30]. At these contents of the elastic filler, they are deformed uniformly at the macro level and non-uniformly at the micro level, while up to some degrees of sample elongation, joint stretching of the matrix with the filler occurs [31]. Let us clarify that non-uniform stretching at the micro level is plastic deformation of the matrix in thin layers between rubber particles. At the elastic filler content of 80 wt % or more, the stretching of thin layers of the matrix will proceed plastically, but without the formation of a neck [32].

As can be seen from Fig. 2, an increase in the content of rubber particles from 80 to 90 wt % is expected to lead to a slight decrease in the deformation and strength properties of the rubber plastics.

Figure 2. Histograms of changes in tensile strength (a) and strain (b) of the rubber plastics depending on the HDPE content in the matrix (*—powder, **—granules).

The results obtained allow us to conclude that the content of HDPE in LDPE waste up to 30 wt % or LDPE in HDPE waste up to 50 wt % inclusive does not lead to deterioration of the mechanical properties of the highly filled rubber plastics. Consequently, in the case of obtaining these materials both from crushed rubber waste and from polyethylene waste, there is no need to control the composition of mixed polyethylene waste and, consequently, in the preliminary technological stages of separating LDPE and HDPE. The exclusion of this stage from the general technological process of treatment of polyethylene waste not only simplifies their processing, but also reduces the cost of secondary polyethylene intended for obtaining highly filled composites.

In addition to the deformation and strength properties discussed above, the elastic modulus and residual strain are also important characteristics for safe road surfaces. As can be seen from Table 1, the values of residual strain for the materials containing 90 wt % of EPDM are not high, i.e., rubber plastics behave as rubber-like materials. However, at the lower filler content (80 wt %), the effect of the plastic matrix polymer on these characteristics increases and leads to an increase in residual strain: for LDPE more than two times and for HDPE almost six times.

Table 1. Mechanical properties of some rubber plastics containing 80 and 90 wt % of EPDM

The results presented above relate to the rubber plastics obtained using crushed rubber technical waste based on EPDM. It was shown earlier that these particles are characterized by good adhesion to polyolefin matrices, which predetermines joint deformation with the matrix polymer. The destruction of rubber plastics is initiated by the rupture of elastic particles [31].

Hence, to obtain highly filled rubber plastic materials with a maximum strain of more than 200% (for example, when using LDPE, Fig. 2b), it is preferable to use rubber crumb based on EPDM. However, the main "source of accumulation" of rubber waste are worn-out automobile tires, the main component of which is isoprene and nitrile rubber. Unlike EPDM particles, rubber crumb obtained by grinding worn-out automobile tires has poor adhesion to polyethylene [30, 31]. In this case, rubber particles peel off from the matrix polymer during stretching of the rubber plastic samples, forming pores that quickly develop into dangerous defects.

For comparison, Table 2 presents the mechanical properties of the rubber plastics based on LDPE-3 containing 80 wt % of EPDM rubber particles and automobile tires (FGRP). In the first case, the strain at break is higher.

One of the ways to improve adhesion between the matrix and the filler is to introduce a compatibilizer. In the case of rubber plastics, this function can be performed by PEVA [32]. It was previously shown that 5 wt % relative to the matrix polymer is the optimal concentration of this additive [33]. To obtain modified rubber plastics, LDPE-3 was used, the content of FGRP was 80 wt %. Table 2 shows the mechanical properties of the resulting materials. When PEVA was used, the deformation properties of the material increased almost two times, but did not reach the level of deformation of the composite with EPDM particles.

The question arises about the suitability of FGRP for obtaining soft injury-safe coatings if the ultimate strain of the material is used as a criterion. Currently, there are no documents regulating the characteristics of injury-safe soft coatings based on rubber crumb. Thus, in accordance with the TU (Specifications) developed at the OOO NPP Ekoresurs (Penza, Russia), the strength of such coatings should be at least 1.9–2.1 MPa with a strain at break of 130–160% [34]. According to these parameters, highly filled rubber plastics based on modified polyethylene and FGRP are suitable for obtaining such coatings.

Table 2. Mechanical properties of different rubber plastics containing 80 wt % of crushed rubber particles

Experimental section

Materials

The work was concerned with HDPE of grades F 3802B (HDPE-1) produced by OOO Stavrolen and 277-73 (HDPE-2) produced by OAO Kazanorgsintez, as well as LDPE of grades 10204-003 (LDPE-1), 15803-020 (LDPE-2), 16803-070 (LDPE-3), and 10803-020 (LDPE-4) produced by OAO Kazanorgsintez. In addition, the mixtures with different LDPE/HDPE ratios were used as the polymer matrices. The properties of the used HDPE, LDPE, and their mixtures are presented in Table 3. Ethylene–vinyl acetate copolymer (PEVA) of grade 11306-075 produced by OAO Kazanorgsintez was used as a modifier.