Dielectric properties of various polymers (PVC, EVA, HDPE, and PP) reinforced with ground tire rubber (GTR)
-
Ramon Mujal-Rosas
,
Marc Marin-Genesca
Abstract
Mass production of tires as well as its difficult storage or elimination is a real environmental problem. Various methods for recycling tires are currently used, such as mechanical crushing, which puts vulcanized rubber, steel, and fibers apart. The rubber may be used in several industrial applications such as flooring, insulations, and footwear. The present paper focuses on finding a new application for old used tires [ground tire rubber (GTR)]. To this end, tires dust has been mixed with various thermoplastic polymers such as polyvinyl chloride (PVC), high-density polyethylene (HDPE), ethylene vinyl acetate (EVA), and polypropylene (PP). We have checked the maximum GTR concentration values admitted by these compounds while keeping dielectric properties within acceptable values and therefore remaining suitable for industrial applications in the manufacturing of insulators for electrical wires. In particular, tires dust with particles size p<200 μm has been mixed with the polymers in four different GTR concentrations of 5%, 10%, 20%, and 50% to establish its performance through dielectric tests performed within a range of temperatures from 30°C to 120°C, and with frequencies from 1×10-2 to 3×106 Hz, and analyzing conductivity, permittivity, dielectric loss factor, and relaxations. At last, the fracture surfaces of the composite samples have been evaluated using scanning electron microscopy (SEM).
1 Introduction
The important problem of the accumulation of old used tires [ground tire rubber (GTR)] [1, 2] has driven the efforts of the international scientific community to seek solutions for recovery and reuse. Many plastic materials include elastomers to improve its toughness. In general, a thermoplastic or thermosetting polymer acts as a matrix and the elastomer acts as a dispersed phase [3]. Moreover, as in other two-phase polymer blends, the interfacial compatibility between the components is important for achieving the desired properties. In the case of recycled elastomers, when mixed with thermoplastic polymers, expected compatibility is low. One way to increase the compatibility between components is to reduce the degree of cross-linked GTR by devulcanization [4–6]. Another way to increase the interfacial bond is by making a pretreatment with acid, producing micropores on the elastomer surface, which facilitate a union between phases [6, 7]. However, this method makes the final product more expensive besides having a weak influence on the improvement of the properties of the compound. More significant changes in properties are observed when the size of the reinforcement particles is changed. That option is studied by Mujal and others in several works [8–11]. However, this work studies the comparison of four polymers [recycled polyvinyl chloride (PVC), high-density polyethylene (HDPE), ethylene vinyl acetate (EVA), and polypropylene (PP)], with an only particle size p<200 μm and with four different GTR concentrations (5%, 10%, 20%, and 50%), and analyzes up to which concentration pure polymers can support keeping the initial dielectric and microstructure properties within acceptable values [12–14]. This would allow adding GTR to several industrial processes such as the manufacturing of insulators for electrical wires.
2 Methodology
2.1 Materials
PVC, with empirical formulation (C2H3Cl)n and semideveloped formula (-CH2-CHCl-)n, was supplied by Ercros S.A. (Spain) under the commercial name of Etinox, with a flow rate of 1.35 g/min and a density of 1.225 kg/m3. HDPE Alcudia 4810-B type, with empirical formulation (C2H4)n and semideveloped formula (-CH2-CH2-)n, manufactured by REPSOL, is white, with a flow rate of 1 g/min at 190°C and a density of 960 kg/m3. EVA Alcudia PA 539 type, also manufactured by Repsol, counts with 18% vinyl acetate, with empirical formulation C4H6O2 and semideveloped formula CH3-COO-CH=CH2, and a 82% ethylene, with empirical formulation C2H4 and semideveloped formula H2C=CH2; its flow rate is 1.35 g/min and its density is 960 kg/m3. PP Isplen® 099 K2M type, with empirical formulation (C3H6)n and semideveloped formula (-CH2-CH(CH3)-)n, is manufactured by Repsol, also white, with a melting temperature of 165°C, a flow rate of 55 g/10 min, and a density of 902 kg/m3. Finally, the old used tires (GTR) with particles sizes lower than 700 μm were supplied by Alfredo Mesalles (Spain). It has been verified, by thermogravimetric analysis, that carbon black content was about 35%. The original GTR was separated by sieving into three categories of particles: p<200 μm, 200 μm<p<500 μm, and p>500 μm, and only particles of size p<200 μm have been used for this work. The chemical structures of PVC, HDPE, EVA and PP are shown in Figure 1.
Chemical structures of polymers used for the present analysis. (A) PVC, (B) HDPE, (C) EVA and (D) PP.
2.2 Preparation of the compound
The recycled tire powder was dried in an oven at 100°C for 24 h. Six samples of polymer/GTR compounds with a composition of 5%, 10%, 20%, and 50% GTR were prepared for the selected particle size. The mixing process was done with a Brabender machine, at different temperatures and with limited mixing time (195°C and 5 min for PVC, 153°C and 4 min for HDPE, 105°C and 4 min for EVA, and 165°C and 4 min for PP), to prevent degradation of polymers. Polymer/GTR laminates (170×170×2 mm3) were obtained by using a Collin Mod hot plates press P 200E at 100 kN for 10 min at 210°C (PVC), 170°C (HDPE), 120°C (EVA), and 180°C (PP). The cooling stage was done with a closed water circuit, which was held in the same press and at the same pressure for 5 min. Samples for testing were properly set up according to specifications of ASTM D-412-98 standard.
2.3 Scanning electron microscopy (SEM)
SEM was used to analyze the fracture surface of those broken samples in tensile deformation tests. It is possible to analyze the effects of the filler in the matrix by observing the environment of the reinforcement particles. Some images of the samples were analyzed according to the compound and GTR concentration. A Jeol 5610 microscope was used, and the samples were previously coated with a thin layer of gold to increase conductivity.
2.4 Dynamic-electric analysis (DEA)
Dielectric spectroscopy was used to measure some electrical properties of the studied samples such as permittivity, loss factor, and conductivity as a function of frequency. The analysis of these measurements provided information about structural relaxations and conduction processes that take place in the compounds and the effect of the GTR particles on them. DEA measurements were also carried out only with particles smaller than 200 μm, because they offer better results in all previous trials. Dielectric parameters and magnitudes were measured by means of a BDS40 equipment, which incorporates a Novocontrol Novotherm temperature sensor, and using a 2 cm diameter compression mould. The measurements were taken in a frequency range between 1×10-2 and 3×106 Hz, with a temperature between 30°C and 120°C, at a rate of 3°C/min, by using parallel plate sensors.
3 Results and discussion
3.1 Morphology
Some SEM micrographs of the fracture surface of the tensile deformation test of polymer/GTR tubes are shown in Figure 2. Two pictures are shown for a particle size p<200 μm, for each polymer/GTR compound, and two extreme concentrations of 10% and 50% [15–18].
SEM micrographs of polymers/GTR compounds for some polymer/GTR concentrations and particles sizes <200 μm.
(A) PVC/GTR-10%, (a-bis) PVC/GTR-50%; (B) HDPE/GTR-10%, (b-bis) HDPE/GTR-50%; (C) EVA/GTR-10%, (c-bis) EVA/GTR-50%; and (D) PP/GTR-10%, (d-bis) PP/GTR-50%.
In all cases, GTR particles do not reach the degradation temperature when mixing, and it is possible to see those particles dispersed in the homogeneous polymer matrix, which reaches the melting temperature. The result is a mass of microgranules, with a degree of dispersion that depends on the mixing time and temperature and that does not provide cohesion between phases.
Generally, for low concentrations of 10% GTR (Figure 2A–D), reinforcement particles are integrated and covered by the matrix. The topology in the amorphous region of matrices shows good interfacial adhesion, including interconnections between crystalline zones. Small areas are cleanly cut, which indicates that the particles have broken before coming away in the tensile test. We can also see how superstructures with homogeneous morphologies are made up. A few gaps appear in the contour of the particle, and some polymer fragments are dispersed on the surface of the particle to adhere to it.
In contrast, for high concentrations of 50% GTR (Figure 2a-bis to d-bis), the morphological structure of materials is affected, and cracks in the matrix increase and worsen the interfacial adhesion. Adherence varies significantly when adding the reinforcement particles (GTR). In this case, the percentage of polymer is not enough to wrap the GTR particles, so the bonding becomes more difficult, and huge cracks and pores appear in the contour of the particles. GTR particles are clean and easy to remove, so the fracture occurs through the interface of the matrix. Moreover, the higher concentrations in GTR are, the more the possibilities of particles agglomeration exist, the agglomerate acting as a large size particle.
For intermediate concentrations of GTR in the matrix, materials are affected by parameters associated to existing ordered and unordered regions, which depend on the level of crystallinity of the matrix polymer. Thus, the final crystallinity of the compound will depend on such factors as the size and distribution of crystalline lamellae, the structure of each matrix, the relative content of the interface region of materials, or the level of stereoregularity in the matrix.
With these concentrations, different levels of linkage appear between components. For concentrations up to 20% in GTR, the interfacial cohesion is still acceptable, as shown by the calorimetric and mechanical properties of the compounds, whereas, with percentages higher than 40% in GTR, the particles begin to show significant discontinuities on their surface with quite large pores and cracks that weaken their mechanical properties.
3.2 Electrical properties
Comparative dielectric analyses have been performed at different frequencies and temperatures for all types of compounds, with GTR particles with sizes p<200 μm, and with the four different concentrations of GTR that are analyzed. By means of the dielectric spectroscopy technique, samples are subjected to a variable frequency AC voltage, and the material’s dielectric magnitudes are derived from the measurement of the impedance. In AC regime, relative permittivity is a complex number whose real part ε′ is proportional to the stored energy in each cycle, while the imaginary part ε″ is proportional to the lost energy, and it is known as the loss factor. Through the analysis of real permittivity and loss factor with frequency and temperature, relaxations of the material might be identified. Therefore, it is possible to identify some information about the structure, transitions with temperature, the effect of addition of reinforcement particles, etc., of the sample. For one single material, observed relaxations are called γ, β, α, and ρ, according to the order in which they appear when going from low to high temperatures. In the range of the studied frequencies and temperatures, only β and α relaxations appear. However, this refers only to the order of appearance for a given material. The origin and properties of each β and α relaxation depend on the polymer.
3.2.1 Conductivity
Figure 3A–D shows the results of conductivity for the different compounds with the four GTR concentrations (5%, 10%, 20%, and 50%), at 30°C, and according to the frequency. Differences in conductivity at low temperatures, depending on the type of material and on the concentration, are lower than one order of magnitude. However, there is an increase of conductivity depending on the frequency in all cases. Thus, for low frequencies (0.01 Hz), the two amorphous PVC [11] and EVA compounds show conductivity by 1×10-15 S/cm, while HDPE (semicrystalline) [13] and PP (more crystalline) compounds show conductivity by 1×10-16 S/cm. As frequency increases (1×107 Hz), GTR concentration is not so important, and the values of conductivity for PVC/GTR and EVA/GTR are by 1×10-7 S/cm, while values for HDPE/GTR and PP/GTR are by 1×10-8 S/cm.
Conductivity (σ) at 30°C related to the frequency.
(A) Compound+5% GTR, (B) compound+10% GTR, (C) compound+20% GTR, and (D) compound+50% GTR: ■, EVA; ◯, HDPE; Δ, PP; ▽, PVC.
If temperature is increased up to 120°C (Figure 4A–D), the performance of compounds differs according to their crystallinity. Thus, for amorphous materials (PVC/GTR and EVA/GTR), two clear areas appear according to the frequency: one of them at low frequencies up to about 100 Hz and another one at frequencies above 100 Hz. Conductivity keeps constant at low frequency areas, and it increases as the content of GTR increases (at 0.01 Hz). However, at 100 Hz and above, a linear dependence is observed on a logarithmic scale.
Conductivity (σ) at 120°C related to the frequency.
(A) Compound+5% GTR, (B) compound+10% GTR, (C) compound+20% GTR, and (D) compound+50% GTR: ■, EVA; ◯, HDPE; Δ, PP; ▽, PVC.
The area where conductivity is frequency independent does not appear for crystalline materials (HDPE and PP) for this range of frequencies. A slight change of performance is observed at frequencies about 0.01 Hz. This change of performance is more significant for HDPE/GTR than for PP/GTR, which matches with the fact that this last compound is the most crystalline one.
As described by León et al. [19], the performance fits a sublinear dispersive model equation:
where σ0 is the direct current (DC) conductivity, ω=2πf where f is the frequency, while A and n (which have values between 0 and 1) are parameters that depend on the temperature, the material properties, and the content in GTR. This equation is known as the universal dynamic response, and it is quite usual in highly disordered materials such as the PVC and EVA and also, but not so much, HDPE and PP. The range of frequencies that separate these areas shifts toward higher values with temperature. At 120°C, conductivity at low frequencies has increased by about three or four orders of magnitudes on average for PVC/GTR and EVA/GTR and for all concentrations (from 1×10-15 to 1×10-10 S/cm for PVC/GTR and from 1×10-14 to 1×10-11 S/cm for EVA/GTR). This is mainly due to the increase of conductivity in DC. Conductivity increases by about two or three orders of magnitudes on average (from 1×10-15 to 1×10-13 S/cm for HDPE/GTR and from 1×10-16 to 1×10-13 S/cm for PP/GTR) for the more crystalline materials (HDPE and PP). The increase of conductivity is much lower for high frequencies, and it is lower than one magnitude in all cases (from 1×10-7 to 1×10-6 S/cm). In the case of all compounds, conductivity increases with the carbon black present in GTR. However, the conductivity behavior with the carbon black usually shows an inflection point at which it asymptotically approaches the limit conductivity, which differs depending on the material. In the case of the PVC/GTR compound at high temperatures, after an initial increase of conductivity values with the GTR, a subsequent decrease can be found. It means that GTR affects the PVC conductivity at high temperatures negatively. In the range of concentrations in which there is a strong conductivity dependence on the carbon black, this effect cannot compensate the increase of conductivity. For concentrations higher than 40% in GTR, the contribution of carbon black reaches the limit and its adverse effects on conductivity remain. By contrast, in all compounds except for PVC/GTR, the increase in conductivity in DC does not exceed 10-10 S/cm, and it is far from those values of semiconductor materials. Finally, the addition of GTR increases the DC conductivity to acceptable values for antistatic applications (from 10-9 to 10-14 S/cm [20]).
3.2.2 Permittivity and loss factor
Figure 5A–D shows the results of real permittivity (ε′) for the different compounds with the four GTR concentrations (5%, 10%, 20%, and 50%), at 30°C, and according to the frequency.
Dielectric permittivity at 30°C related to the frequency.
(A) Compound+5% GTR, (B) compound+10% GTR, (C) compound+20% GTR, and (D) compound+50% GTR: ■, EVA; ◯, HDPE; Δ, PP; ▽, PVC.
In the case of polar materials (PVC and EVA), permittivity is affected by the content of GTR; it increases when concentration increases, and this increase is more noticeable at lower frequencies and concentrations higher than 20% in GTR (permittivity is 3.5 for both PVC/GTR and EVA/GTR, at 0.01 Hz and 5% GTR, while it is 10 and 13 for PVC/GTR and EVA/GTR, respectively, for concentration of 50% GTR and at the same frequency). In contrast, frequency negatively affects permittivity because its value decreases due to the dielectric dispersion, and the differences between compounds decrease as well (at a frequency of 1×107 Hz and concentrations of 5% and 50% GTR, the values of permittivity are 2.8 and 4.8 for PVC/GTR and 3 and 5.5 for EVA/GTR).
For nonpolar materials (HDPE and PP), real permittivity increases as the content of GTR increases, this increase being more significant at low frequencies. For 0.01 Hz and 5% GTR, permittivity is 2.8 and 2.4 for HDPE/GTR and PP/GTR, respectively; for 50% GTR and the same frequency, permittivity is 11 and 16 for HDPE/GTR and PP/GTR, respectively. Regarding the influence of frequency, permittivity decreases as frequency increases mainly due to the dielectric dispersion. Because HDPE and PP are not polar materials, only GTR contributes to that phenomenon, so this is less significant for low concentrations of GTR and becomes zero for pure materials. At a frequency of 1×107 Hz and concentration of 5%, permittivity is 2.1 for both HDP/GTR and PP/TR; for concentration of 50% GTR and the same frequency, permittivity is 5 and 3 for HDPE/GTR and PP/GTR, respectively.
Figure 6A–D shows the results for the imaginary permittivity (ε″) in the different compounds with concentrations of 5%, 10%, 20%, and 50% GTR at 30°C, in function of the frequency. In the case of polar materials (PVC and EVA), loss factor increases with the concentration of GTR (at a frequency of 0.01 Hz and with a concentration of 5% GTR, the values of losses are 0.15 and 0.3 for PVC/GTR and EVA/GTR, respectively, while they are 1 and 2 for a concentration of 50% GTR at the same frequency). With regard to the dependence on the frequency, in all the PVC/GTR compounds, the loss factor decreases rapidly in the region of low frequencies up to 10 Hz, and then it increases and reaches a maximum located between 1×104 and 1×105 Hz. This peak in the loss factor spectrum can be related to the β relaxation, which is linked to the local motions of the main chain. β relaxation in PVC has been studied previously by Ishida [21].
Dielectric loss (ε″) at 30°C related to the frequency.
(A) Compound+5% GTR, (B) compound+10% GTR, (C) compound+20% GTR, and (D) compound+50% GTR: ■, EVA; ◯, HDPE; Δ, PP; ▽, PVC.
For EVA/GTR compound, at high frequencies and concentrations not higher than 20% GTR, there is an increase in dielectric losses with frequency, which can be associated to the α relaxation of EVA [22] when taking into consideration the temperature and frequency. This relaxation might be related with the segment motions in the amorphous phase above the glass transition temperature, which takes place below the 0°C.
For nonpolar materials (HDPE and PP), loss factor increases with the content of GTR, this increase being more significant at low frequencies. For 0.01 Hz and 5% GTR, the loss factor is 0.035 and 0.028 for HDPE/GTR and PP/GTR, respectively, while, for 50% GTR and the same frequency, it is 1.05 and 1. The loss factor dependence on the frequency is due to the contribution of conductance
Figure 7A–D shows the results of dielectric permittivity (ε′) for the different compounds with the four GTR concentrations (5%, 10%, 20%, and 50%), at 50 Hz, and as a function of the temperature. Generally, the increase of permittivity as temperature increases is due to the contribution of two mechanisms: The first one is an increase of mobility of the macromolecules, with a decrease in density to increase the permittivity. The second one is a difference in thermal expansion between the different areas of the polymer that reduces it [24]. However, above the glass transition temperature Tg, the first contribution is much higher than the second one, so the overall permittivity increases. The presence of GTR particles emphasizes both mechanisms, with the result of both contributions significantly increasing the permittivity. This performance, as evidenced by Saad et al. [25] and Tsangaris et al. [26], is very similar in most of the compound materials loaded with reinforcement, with a much higher conductivity than that of the matrix itself.
Dielectric permittivity (ε′) at 50 Hz related to the temperature.
(A) Compound+5% GTR, (B) compound+10% GTR, (C) compound+20% GTR, and (D) compound+50% GTR: ■, EVA; ◯, HDPE; Δ, PP; ▽, PVC.
By analyzing polar materials (PVC and EVA), it is observed that there is a slight increase of permittivity for PVC/GTR compound up to the 70°C depending on the content of GTR (for 30°C, values of permittivity are 3.2 for a 5% GTR and 7 for a 50% GTR, and these values increase to 3.4 and 7.2 at 70°C and the same conditions). From this point, there is an abrupt change in the slope, which increases significantly between 80°C and 110°C (values are 10.2 and 10.4 at 110°C and concentrations of 5% and 50% GTR, respectively) and returns, from this temperature, to the original tendency. The EVA/GTR compound performs in a different way, because its permittivity hardly varies with temperature (less than 1 unit in all cases), although it varies with the concentration of GTR (from 3.5 to 8.5 for 30°C and concentrations of GTR of 5% and 50%, respectively) to undergo a transition between two stable regimes subsequently. This transition starts at 80°C, its inflection point is located between 90°C and 100°C, and it ends at 110°C. Note that this temperature range matches, approximately, with the melting range of the EVA.
For nonpolar materials (HDPE and PP), we may see a slight increase of dielectric permittivity depending on the concentration of GTR. Thus, for extreme concentrations of 5% and 50% GTR, the values of permittivity are 2.5 and 7.8 for HDPE/GTR, respectively, and values are 2.25 and 5.5 for PP/GTR, respectively. Generally, permittivity slightly decreases when temperature increases, showing a weak dependence between them.
Figure 8A–D shows the results of imaginary permittivity (ε″) for the different compounds with the four GTR concentrations (5%, 10%, 20% and 50%), at 50 Hz, and as a function of the temperature.
Dielectric losses (ε″) at 50 Hz related to the temperature.
(A) Compound+5% GTR, (B) compound+10% GTR, (C) compound+20% GTR, and (D) compound+50% GTR: ■, EVA; ◯, HDPE; Δ, PP; ▽, PVC.
Concerning polar materials (PVC and EVA), we may see that loss factor increases from 70°C for PVC/GTR (0.045 for 5% GTR and 0.35 for 50% GTR), and it shows a relaxation around 90°C (with values of loss factor of 1.5 for 5% GTR and 2 for 50% GTR) in coincidence with the inflection point of the real part. This phenomenon is more noticeable with low GTR concentrations, as it happens for the β relaxation of PVC. The reason for this performance of the PVC/GTR compound is that, with temperatures below the glass transition temperature (where the amorphous fraction of the polymer changes from a glassy state to a rubbery state), polymer chains are frozen and temperature hardly causes any increase in mobility. In contrast, near the glass transition, rearrangement of main chains is carried out on a large scale, resulting in α relaxation, which can be observed at different temperatures depending on the frequency we work at. Calorimetric measurements suggest that glass transition temperature (Tg) is around 80°C. Also, it is worth to note that, at temperatures above the glass transition temperature, there is an increase of dielectric loss as a function of temperature, and these temperatures are higher than the α relaxation temperature of PVC. It has been found that this phenomenon increases markedly when frequency decreases, but there are different contributions affecting the dielectric response (space charge injection, electrode polarization, conduction of absorbed impurities, etc.), hiding the relaxations that are present in this spectral region [26–28].
Concerning the EVA/GTR compound, its performance is quite similar. Loss factor decreases to a minimum of 60°C (0.02 for 5% GTR and 0.3 for 50% GTR), and it starts to increase with temperature up to values of 0.1 and 0.7 at 120°C and concentrations of 5% and 50% GTR, respectively. As the EVA/GTR relaxation map shows [22, 29], the end of the α relaxation is reached at low temperatures, whereas the effect of conductive processes in the loss factor ε″ is observed at higher temperatures.
For nonpolar materials (HDPE and PP), we can see a slight increase of the loss factor depending on the concentration of GTR. Thus, for the HDPE/GTR compounds, the loss factor ε″ hardly increases as temperature increases, and its value keeps almost the same. For extreme concentrations of 5% and 50% GTR, values are 0.01 and 0.95, respectively. Because the GTR is more polar and conductor than the HDPE, dielectric relaxation of the polymer matrix is hidden by the properties of the filler. The interfacial relaxation phenomenon in heterogeneous materials is usually found at very low frequencies, so it is not visible in measurements at low temperatures for the examined range of frequencies. Because these are thermally activated processes, they should appear for this range of frequencies and higher temperatures.
Concerning the PP/GTR compound, its performance shows a slight initial drop of the loss factor ε″ to a temperature of 70°C, and it goes up from this temperature on, remaining almost constant and with a value of 0.01 for 5% GTR and a value of 0.2 for 50% GTR, during the rest of the analyzed spectrum. According to the pure PP relaxation map, the low temperature region shows the end of the β relaxation. However, α relaxation, which is related with the crystal area, cannot be appreciated. The increase observed in the region of high temperatures is due to the conduction relaxation.
4 Industrial applications
In view of the results we got with the different compounds, without any prior pretreatment with acid or devulcanization of GTR, the best results are achieved with concentrations of 10% GTR, and although the compatibility is low, there are no problems in terms of processing or mechanical shaping. Therefore, the EVA/GTR-10% compound is probably the compound with best results as a whole. With its mechanical and dielectric properties, there are some possible applications in the dielectric field, although always considering materials with low specific requirements, among which we may find electrical cable covers, low voltage electrical insulators for electric fences, electrical cable pipes and trays, universal joints for power cables, spacers for electrical power lines, filler for electrical applications, and footwear for industrial use with electrical insulating characteristics. In Table 1, the properties of the EVA/GTR-10% compound and the requirements for such industrial applications are shown as well as the related official regulations.
Mechanical and electrical properties (50 Hz) of the EVA/GTR-10% compound and the requirements for some industrial applications with the corresponding official regulations.
| Tensile strength (MPa) | Elongation at break (%) | Conductivity (S/cm) | Loss tangent | Official regulations | |
|---|---|---|---|---|---|
| EVA/GTR (10%) | 12.75 | 520 | 7.74×10-13 | 0.008547 | |
| Cable covers | >12.5 | >50 | <10-12 | <3.5 | UNE-HD 632, UNE-EN 60811-4-1, UNE-EN 60811-1-1 (EN 60811-1-1, IEC 811-1-1) |
| Insulators for electric fences | >12.5 | >350 | <10-11 | <3.5 | ITC-BT-39; 22, 23, 24; UNE EN 60335-2-76; IEC 60335-2-76 |
| Cable pipes and trays | >15 | >40 | <10-11 | <5 | UNE EN 61537; UNE EN 50085-1 |
| Universal joints | >12.5 | >35 | <10-12 | <4.5 | IEC 60840, UNE HD 628 |
| Spacers | >17.2 | >30 | <5.5×10-5 | <3.5 | ANSI/IEEE C2, IEC 61854 (273 and 278) |
| Filler | >12.5 | >35 | <10-12 | <5 | UNE 53 602; UNE 53 510; UNE-HD 632; UNE-EN 60811-4-1 |
| Footwear | >10–12 | >35 | <10-9 | <3.5 | UNE-EN ISO 20345/6/7:2005; UNE 53510 |
5 Conclusions
In the present work, with GTR particles sizes smaller than 200 μm, the analyses of SEM micrographs show that the filler affects the microstructure of the polymeric compound by causing an increase of cracks in the matrix. This worsens the interfacial adhesion and leads to the formation of agglomerates that cause cracks and pores of significant size on the surface. However, it has been found that these effects are not significant for low concentrations of GTR.
As for the dielectric tests, the addition of GTR causes an increase of conductivity, permittivity, and dielectric loss factor in all the compounds. The compounds based on polar polymers (PVC and EVA) present higher values of conductivity, permittivity, and loss factor than nonpolar ones (HDPE and PP), as expected, especially for low concentrations of GTR and at high temperatures. Regarding the dependence on the frequency, conductivity shows a dispersive sublinear performance in all the cases. On the contrary, loss factor increases with the reciprocal of the frequency due to interfacial relaxation phenomena and the effect of the conductivity, both processes being enhanced by the addition of GTR. At high frequencies and low temperatures, permittivity and loss factor show relaxations related with structural changes and molecular motions specific of any polymer matrix. Finally, all the compounds display a reduction in insulation characteristics when increasing the content of GTR and improving, by contrast, the antistatic properties, particularly for concentrations higher than 20% GTR.
The analyzed compounds were exclusively obtained from recycled polymers without any prior pretreatment with acids, which have been proven as ineffective and costly. The analysis of the results shows us that 10%–15% GTR concentration is the limit concentration value, with particles sizes lower than 200 μm, for keeping dielectric and structural properties within acceptable values. Other methods such as prior devulcanization of GTR should be tested to verify whether these values might be increased up to 20%–25% in GTR. This would expand its use in many industrial processes.
Acknowledgments
The authors would like to thank the Ministry of Science and Technology of Spain and the Agència de Gestió d’Ajuts Universitaris i de Recerca of Catalonia for the funding to develop the projects MAT 2007-64569, MAT 2008-06 284-C03-02, and 2009-SGR-1512. The authors also thank the Nuclear Physics Department of the Universitat Politècnica de Catalunya, Terrassa section, with special mention of Professor M. Mudarra and J. Belana, for their support in carrying out the dielectric tests.
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Articles in the same Issue
- Frontmatter
- Original articles
- Dielectric behaviors of carbon nanotube/silicone elastomer composites
- Investigation of damping and toughness properties of epoxy-based nanocomposite using different reinforcement mechanisms: polymeric alloying, nanofiber, nanolayered, and nanoparticulate materials
- Dielectric properties of various polymers (PVC, EVA, HDPE, and PP) reinforced with ground tire rubber (GTR)
- Research on vacuum brazing of W-Cu composite to stainless steel with Cu-Mn-Co brazing metal
- Production and wear property of electroless Ni-plated B4C-AstaloyCr-M composites
- Compressibility and solid-state sintering behavior of W-Cu composite powders
- Assessment of nano-TiO2 and class F fly ash effects on flexural fracture and microstructure of binary blended concrete
- Use of glass waste as mineral filler in hot mix asphalt
- Effect of latex powder and glass fibers on the performance of glazed hollow bead thermal insulation materials
- Experimental and finite element studies on buckling of skew plates under uniaxial compression
- Analytical and experimental investigation of tensile properties of cross-ply and angle-ply GFRP composite laminates
- Modeling of tensile and bending properties of biaxial weft knitted composites
- Research on design rules for composite laminate
- Individual humeral head replacement by C/C composite implants coated with hydroxyapatite via rotation plasma spraying
- Geopolymer ferrocement panels under flexural loading
