Utilization study on red brick waste as novel reinforcing and economical filler for acrylonitrile butadiene rubber composite

Red brick waste (RBW) powder was used as a reinforcing filler for acrylonitrile butadiene rubber (NBR) for preparing eco-friendly composites. The main constituent of this waste is silica which is considered to be one of the traditional fillers used in the rubber industry. So it was worthwhile to get (RBW) in a suitable manner by using this waste as a filler for NBR. Physicomechanical properties were evaluated, including tensile strength, elongation at break, hardness shore A, and change in mechanical properties after thermal-oxidative aging for seven days at 90 °C. In addition, equilibrium swelling in toluene for prepared composites was also included. Moreover, cross-linking density was measured according to Flory Rehner's equation. From physicomechanical measurements, it is observed that the tensile strength and hardness (shore A) increased with increasing the waste filler content because of the red brick filler's stiffness. In contrast, the elongation at break and equilibrium swelling degree decreased with increasing the waste filler content. The morphology of the polymeric composites was monitored using a scanning electron microscope which reflects the uniform distribution of the filler inside the NBR matrix. The electrical and magnetic properties of NBR/RBW composites were evaluated. The electrical measurements revealed that the values of ε′ increase by increasing red brick waste while ε'' decreases by increasing filler content. The dc conductivity σdc values lie in the order of 10–11 S/cm. This finding recommends that such composites could be used for insulation and antistatic applications. In addition, the magnetic measurements show superparamagnetic behavior in NBR/RBW composites. Therefore, the obtained composites be used as electromagnetic composites based on electrical and magnetic measurements.


Introduction
The construction fields contribute to the social and economic growth of many countries. Tremendous development in the construction field, such as the development of informal rural areas and the establishment of global road networks, has been witnessed worldwide, especially in the past few years (Habibi and Nasir 2020). This tremendous development in the construction field has been accompanied by a noticeable increase in actual waste, which represents a challenge facing the Egyptian government (Daoud et al. 2020;Azmy and El Gohary 2017;Abdelhamid 2014). Therefore, the issue of the safe disposal of these wastes is of great importance for most countries worldwide. Many efforts by many researchers have been made to dispose of these wastes in appropriate ways in an attempt to reduce costs while preserving the environment (Garas et al. 2001;NSWMP 2013).
Construction and demolition waste (C&Dw) material mainly consists of concrete, masonry bricks, limestone, sandstone, metal, and wood, depending on the construction type (Bossink and Brouwers 1996). In Egypt, (C&DW) amount was estimated as ten thousand tons daily, accounting for 4.5 million tons per year (El Ansary et al. 2004). Red brick is one of the essential building materials that is broadly used all over the world. The amount of red brick waste increases not only from construction and demolition but also during its manufacturing process. These wastes are disposed of by traditional methods by transporting them to suitable disposal areas, often landfills (Li et al. 2016;Kartam et al. 2004;Merino et al. 2010). The traditional methods of disposing of these wastes represent a tremendous burden on countries, as countries spend vast amounts of money on transporting and burying these wastes. These wastes also cause severe damage to the surrounding environment as dust is easily generated during transportation (Merino et al. 2010).
The work aims to provide novel, cheap and eco-friendly methods for recycling solid waste based on demolition and construction processes. Many researchers have focused on the incorporation of these solid wastes into polymer matrices and composites because it allows for the approach to the sustainable development of industrially applicable waste (Chandrappa 2012). Acrylonitrile-butadiene rubber (NBR) is a polar, synthetic rubber used in many different applications, including automobile, aerospace, and petroleum industries, due to its good properties as mechanical properties in addition to its chemical resistance furthermore, high permittivity ε′ (Rozik et al. 2017;Hu et al. 2020;Shafik et al. 2020). The objective of this study is to use red brick waste powder (RBW) as a filler for acrylonitrile butadiene rubber instead of traditional fillers. In addition, it is intended to keep the environment from these wastes and manufacture low-cost, eco-friendly composites that exhibit both desirable mechanical and electrical properties in addition to their magnetic properties.

Assessment of red brick wastes
Red brick waste (RBW) was ground through a jaw crusher, then in a mill grinder and sieved using USA standard testing sieve No. 120 to get the same particle size. X-ray fluorescence (XRF) analysis was used to characterize the red brick waste powder. Type: Axios, Sequential mod. (WD-XRF), PA Nalytical 2005 obtained from the Netherlands.

Preparation and characterization of NBR/RBW composites
Red brick waste powder (RBW) was incorporated into NBR composites with different content starting from 0 to 40 phr. The compounding process was achieved using a laboratory two-roll mill according to ASTM D3182-07. The formulation of NBR/RBW composites is illustrated in Table 1.
Curing of the samples was done at 152 ± 1 °C until they developed a 90% increase in torque using Moving Die Rheometer (MDR 2000). An electrical hot press was used to compress and mould all the samples depending on their respective cure time (Tc 90 ).
Mechanical properties of vulcanizates were determined using an electronic Zwick tensile testing apparatus (style Z010, Germany), per ASTM D412 standard. The thermal oxidative aging was assessed at 90 ± 1 °C using an oven for seven days based on the ASTM no: D 572-04, 2010. Zwick Roell durometer machine coming from (Germany) was used for measuring Shore A hardness of the NBR composites.
The cross-linking density was measured through the willpower of equilibrium swelling (ASTM: D471-06). Firstly, the equilibrium swelling in toluene (Q%) was measured and calculated using the equation below: Ws: weight of the swelled sheet.
Wd: weight of the dried sheet.
In addition, the Flory-Rehner equation was used to compute the cross-linking density: where: ρ = rubber density, V s = solvent molar volume, X is the interaction parameter of NBR and is equal to 0.39, and Vr is the volume fraction of swollen rubber obtained from masses & densities of rubber and solvent. Crosslinking degree (ν) in (mol/cc) is calculated as: A scanning electron microscope was performed using the Quanta instrument (FEG250, FEI, Hillsboro, Oregon, USA).

Electrical properties of NBR/RBW composites
Novo control Alpha Analyzer (GmbH concept 40, tanδ > 10 −4 ) was used to carry out the permittivity and the dielectric loss besides the electrical conductivity. The measurements were carried out over a frequency range from 10 −1 to 10 7 Hz. Two gold parallel plate capacitors were used as a measuring cell with a diameter of 10 mm. This technique has the facility to explore molecular variations and charge movement in broad frequency ranges (Saied et al. 2022).

Magnetic properties of NBR/RBW composites
Vibrating sample magnetometer (VSM) A vibrating sample magnetometer model, Lakeshore Model 7410, USA, was used for the magnetic properties measurements. This system is set with a 10-inch electromagnet giving magnetic fields up to 3.1 T, accuracy = ± 1%, sample size = 1 in and variable temperature vary from (CCR) = ~ 20 to 450 K (El-l-Nashar et al 2006).

Results and discussion
Red brick waste powder chemical composition Table 2 shows the RBW chemical composition resolute by XRF. The chemical composition of RBW powder consists of many oxides but mainly encloses high ratios of three oxides: SiO 2 , Al 2 O 3, and Fe 2 O 3 . RBW powder can be used as a filler in rubber because RBW powder contains a high ratio of silica, one of the essential fillers used in the rubber industry. The red colour of the bricks is also due to the presence of iron oxide in its chemical composition, which is the privilege responsible for the magnetic properties added to the composite.

Rheometer characteristics of NBR/RBW composites
RBW powder effect on the rheological conduct and curing efficiency of the NBR composites were assessed based on rheometric measurements as shown in Table 3. The curing parameters include minimum torque (M L ), which measures the hardness and viscosity of the unvulcanized rubber and reflects its processability. The maximum torque (M H ) shows the stiffness of the composites, and the difference between torques (ΔM) expresses the composites' cross-linking density and shows the curing process state.  (Hosseini and Razzaghi-Kashani 2014). Also, the presence of red brick powder waste caused a noticeable enhancement of the (ΔM) value, which increased from 6.4 d.Nm for NBR up to 11.23 d.Nm for NBR/Rbw40. This increase confirms a valuable enhancement in the cross-link density of those composites. The RBW powder incorporation contributed to a significant decline in values in both ts 2 and Tc 90 . This decline may be due to the catalytic motion of the RBW powder waste in elastomer systems cured with elemental sulfur. Besides, the cure rate index for NBR composites was measured according to the following equation The cure rate index for NBR composite was 19.42 min −1 and increased with increasing the filler loading until it reached 20 phr, which was 22.22 min −1 , following a decline in the values of the cure rate index above this ratio. CRI = 100∕ Tc 90 − ts 2

Thermal oxidative aging of NBR/RBW composites
Thermal oxidative aging is an essential test for materials that can affect their end quality [14]. Therefore, the effect of incorporation of RBW on the thermal aging resistance of NBR composites was assessed through the variations in the retained values of both tensile and elongation at breaks. Figure 2a, b points up the retained tensile and elongation at break rate during seven days at 90 °C for NBR and NBR containing different content from red brick waste. From this figure, it is observed that the highest retained value of tensile strength was achieved by NBR/RBW 40 (89.25%), and the order of retained tensile strength values for NBR/RBW composites is given as follows: The uppermost retained amount of elongation at break was detected in NBR/Rbw40 (89.1%), while the lowest retained value was (76.2%) for NBR. Overall, it can be stated that incorporating RBW into NBR enhances the composite's thermal stability, which will be convenient for the end product life.

Hardness measurement
Hardness (shore A) is the material's resistance to permanent deformation resulting from mechanical indentation or Figure 3 illustrates hardness (shore A) and RBW filler content. Hardness gradually increased with the RBW content (shore A) from 55 for blank NBR to 68 for NBR/ RBW 40. This increase is a logical feature because of the stiffness of the red brick filler, and these results are consistent with those of mechanical measurements (He et al. 2018;Patel et al. 2021).

Swelling and cross-linking density measurements
The addition of RBW powder fillers to NBR composites may affect the equilibrium swelling and cross-linking density. Figure 4a illustrates the relationship between swelling degree versus red brick filler content. This figure shows that as the RBW concentration increases, the solvent absorption decreases, and the equilibrium swelling goes from 191.02 for NBR to 143.8 for NBR/Rbw40. This finding suggests an interaction between NBR and filler, which will be supported by an increase in relaxation time τ 3 in the dielectric part (Abd El-Aziz et al. 2022). On the other hand, Fig. 4b illustrates the correlation between crosslinking density versus red RBW content, which shows an increase from 2.08 × 10 -4 for NBR to 3.52 × 10 -4 for NBR/ Rbw40. This increase could be explained as RBW powder fine distributed inside the matrix, which had enough contact with the composites, causing much more filler/rubber interactions and less filler/filler interactions (Hosseini and Razzaghi-Kashani 2014).

Dielectric measurements of NBR/RBW composites
The dielectric parameters ε′ and ε″ were measured over the frequency range of 10 0 to 10 7 Hz at room temperature, and the data was depicted in Fig. 5. The values of ε′ are found to increase by increasing RBW while ε" it is apparent that it decreases by increasing RBW reflecting improvement in the insulation properties of NBR. The increase in ε′ values comes from the surface area of the RBW dispersed in the NBR matrix, which physically interacts with it, increasing ε′ values (Pradhan et al. 2008). While on the other hand, adding RBW with less polarity than NBR reduces its loss tangent and, consequently, its ε" values.
This finding is much clearer when both ε′ and ε" are illustrated at f = 100 Hz, versus RBW percentage (see Fig. 6). That figure indicates the composite containing 30 phr RBW is the most promising one as it possesses a high ε′ and a low loss ε" value that fulfils the requirement needed for insulation purposes.
At low frequencies, both the ε′ and ε″are reported high due to charge polarization close to the electrode/electrolyte interface induced by the attendance of free mobile charges in the backbone of NBR (Campbell et al. 2001;Rozik et al. 2021;Ramya et al. 2008). At higher frequencies, NBR dipoles cannot rotate parallel regarding the fast cyclic phase reversal of the electric field, resulting in a temporal lag between the dipole's frequency and that of the electric field (Rozik et al. 2018;Hassan et al. 2020).
For further clarification, the different dielectric relaxation mechanism's effects on the RBW percentage should be studied. For example, the relation of ε″ and the applied frequency (f) given in Fig. 5 reveals more than the relaxation process in the f domain and the well-defined one in the highfrequency range. So it was reasonable to analyze such curves using a computer program based on the various dielectric functions mentioned elsewhere ).
An example of the analyses was given in Fig. 7 for NBR/30 phr RBW. The analyses revile the presence of three relaxation processes fitted by two Fröhlich terms and Havriliak-Negami (HN) one.
The first and second relaxation τ 1 and τ 2 values ascribe the electrode polarization and Maxwell-Wagner-Sillars (MWS) effect (Shafik et al. 2020;Rozik et al. 2015;Hassan et al 2020) are found to be unaffected by the content of RBW. Their values were 0.02 and 2.6 × 10 −3 s, respectively. The third process with relaxation time τ 3 in the order of 10 -7 s is of our interest because it ascribes how the RBW can affect the rotation of the attached groups of NBR. Figure 8 presents the relation between τ 3 and RBW content.
The relation between RBW increases linearly, indicating little physical interaction between NBR and RBW. However, this interaction raises the molar volume of the rotating parts and consequently τ 3 . It is worthy that this interaction The electrical conductivity σ dc was determined from the measured a.c. Conductivity and the obtained data are given in Fig. 9. σ dc values decreased by increasing RBW content which could be the reason for the improvement of the insulation properties of the composite under investigation. Also, it is found that σ dc values lie in the order of 10 -11 S/cm. This finding recommends that such composites be used for insulation and antistatic applications (Huang 2002).

Magnetic properties of NBR/RBW composites
The vibrating sample magnetometer (VSM) technique is a vital tool used to know and provide magnetic properties and the parameters of the nanocomposites with acceptable accuracy relatively fast at room temperature.
The hysteresis loops of NBR/RBW composites are shown in Fig. 10 at different content (10, 20, 30, and 40phr) of red brick waste powder and values of saturation magnetization (MS), remnant magnetization (Mr), and coercively field (Hc) are tabulated in Table 4.
It is seen that the hysteresis loops of the RBW powder, which are filled in the NBR rubber (which is a nonmagnetic  (El-Nashar et al. 2006). NBR/Rbw40 shows a saturation magnetization (Ms) equal to 28.34 × 10 -3 emu/g, and the coercivity field (Hc) equals 224.30. While for NBR/RBW 10, the saturation magnetization (Ms) equals 6.49 × 10 −3 emu/g, and the coercivity field (Hc) equals 135.83.
Also, it can be stated that when the red brick waste powder content increased, the main magnetic parameters obtained from the hysteresis loop: saturation magnetization (MS), coercivity field (HC), and remnant magnetization (Mr) increased. Figure 11 shows the SEM images of NBR, NBR/Rbw20, and NBR/Rbw40 composites. Figure 11a shows that the NBR surface (without filler) is flat, smooth, and less rough, showing a high homogeneity of the polymer matrix and the additive, such as vulcanizing agents. Once the red brick filler is incorporated, the surface roughness increases, whereas Fig. 11b represents acrylonitrile butadiene rubber filled with 20 phr of RBW. It can be observed that the excellent distribution of the filler inside the rubber without any agglomerations is confirmed by increasing the waste filler content to 40 phr. As in Fig. 11c, it can also be seen that the uniform distribution of the RBW inside the NBR matrix supports the increase in mechanical properties by increasing the waste filler content.

Conclusion
According to XRF results analysis, the red brick waste powder can be used as an alternative filler instead of traditional fillers because it contains a high percentage of silica in its chemical composition. The mechanical measurements conclude that incorporating red brick waste powder as filler in acrylonitrile butadiene rubber enhances the tensile strength by increasing the filler content. Also, the hardness (shore A) increased with increasing red brick filler content. In contrast, the elongation at break decreased with increasing the filler content. Furthermore, the electrical measurements revealed that the values of ε′ are found to increase by increasing red brick waste. At the same time, it is apparent that ε" decreases by increasing RBW reflecting improvement in the insulation properties of NBR. In addition, the magnetic measurements revealed that the NBR/RBW composites show a superparamagnetic behavior, and the hysteresis loops depend on the red brick waste powder content filled in rubber. Finally, the authors recommend the possibility of using RBW powder as an alternative to traditional filler. Furthermore, the authors recommend using NBR/RBW composites in antistatic and electromagnetic applications.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Conflict of interest
The authors declare that they have no conflict of interest.
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