Manifestation of Waste Silicate Type Additives and Electron Beam Irradiation on Properties of Sbr/devulcanized Waste Tire Rubber Composites for Floor Tiles Applications


 Virgin styrene-butadiene rubber (SBR) was replaced by devulcanized waste tire rubber (DWR) 50/50 and used as a rubber base for preparing composites to depend on different silicate types at fixed content 40 phr (part per hundred part of rubber). All composites were mixed on a rubber roll mill and then subjected to electron beam irradiation to induce cross-linking at a dose of 100 kGy. Different silicate fillers were used in this study like precipitated silica (PS) 40 phr, waste glass window (WG) - PS 20/20 phr, fly ash (FA)-PS 20/20 phr, and micaosilica (MS)-PS 20/20 phr. Waste silicate was treated with (3-aminopropyl)trimethoxysilane (APTMS) and blended with PS. Mechanical properties were investigated for composites like tensile strength, elongation at break, tensile modulus, and calculation of cross-link density from mechanical. As well as, application for floor tiles included compression set and abrasion resistance measurements. All results indicated an enhancement in tensile strength, modulus, and cross-link density by adding silicate fillers and more enhanced in presence of radiation. For the application of floor tiles, the MS filler gave a good compression set and abrasion resistance followed by other silicate fillers (PS, FA), except WG.


Introduction
The significance of polymer reprocessing has seriously increased through the latest years. Eco-friendly and cost-effective strategies have been utilized for waste management. Rubber is a significant constituent in the civic solid wastes. It has to be reclaimed after discarding [1]. The blending of reclaimed rubber with virgin polymer matrix diminishes the expense of the final product and supports in lessening the environmental contamination. Numerous techniques [2] were tried to lessen this ecological burden by providing well-organized procedures for waste rubber (WR) reprocessing. The presence of worn-out tires in landfills acts as nourishing media for insects to spread diseases. Besides, tire scarps can produce poisonous fumes upon ignition which leads to environmental pollution [3]. Discarded tire rubber as a vulcanized polymer is one of the solid waste pollutants that cannot be disposed of easily with its cross-linked arrangement. Sulfur is one of the used chemicals for rubber vulcanization through composing both S-S and C-S bonds creating a cross-linked matrix. Upon devulcanization, the S-S and C-S bonds are predictable to separate; nonetheless, this does not occur perfectly. For a fruitful devulcanization step, the cross-linked structure needs to separate without chain scission. Devulcanization techniques incorporate mechano-chemical, reclaiming, grinding, devulcanization microwave, and ultrasonic processes. Among these techniques, rubber mechano-chemical devulcanization is utilized. This is achieved by converting the vulcanized or scrap waste rubber mechanically and chemically at high temperatures into a beneficial material ready to be devulcanized and processed [4].
A wide assortment of particulate fillers is utilized in the rubber industry to revise and improve the physical properties of elastomeric materials. The addition of filler frequently prompts an expansion in modulus and significant abrasion and tear resistance. Although the mechanisms of reinforcement are not completely understood, there is an overall agreement about the essential process adding to the stress-strain conduct of the filled vulcanizate [5]. Numerous kinds of fillers are utilized for the improvement of mechanical properties. Carbon black is the most extensively utilized filler in rubber industries owing to its particulate nature and higher surface area, which grants high reinforcing capacity. Nonetheless, these days, more consideration is being paid to nonblack mineral fillers such as silica, owing to the monotonous color and the high costs of carbon black. Looking for lessening the cost of composite, new materials are being considered to supplant absolutely or somewhat the customary ones as a simple economical measure or to grant some required properties. Numerous efforts have been made to utilize silica from the characteristic assets as substitute reinforcing filler in synthetic and natural rubbers in light of cost investments, better dimensional dependability, excessive mechanical properties, and environmental issue. Silica has various hydroxyl groups on the surface, which results in strong filler interaction and absorption of polar materials by hydrogen bonding. In the meantime intermolecular hydrogen bonding between hydroxyl groups on the surface of silica is very strong; it can form tight aggregates [6,7]. The best approach to keep away aggregation is to enhance the compatibility between rubber and silica to lessen the silica migration. Thus, hydrated silica confers better physical properties to polar synthetic rubbers than it doesn't to nonpolar rubbers such as styrene-butadiene rubber (SBR), natural rubber, and so on and is utilized to create colored articles that need high strength properties [8].
New methodologies were explored by researchers to replace carbon black to diminish the utilization of petroleum. This prompts the institution of inorganic fillers such as silica [9] into the rubber, which assists with creating colored products. Silica is the most widely recognized inorganic filler utilized in the rubber industry for reinforcement [10,11], conversely, the isolated silanol groups and hydrogen-bonded silanol groups on the surface of silica decrease its compatibility with non-polar rubber [8]. So, silica was revised with silane coupling agents [11,12] to make it compatible with non-polar rubbers and hybrid filler systems [13] were likewise utilized nowadays.
The reason for consolidating inorganic filler into the polymer network isn't just to accomplish exceptional properties of polymer composites but also to diminish the expense. For ecological protection and sustainable improvement, people have required an excessive effort to manufacture polymer/FA composites that come across the necessity of practical application [14,15].
Blending has to turn out to be the effective technique for growing the utilization of unique materials, and of accomplishing certain necessary properties, by joining the good original properties of individual parts. Rubber blends have established increasing consideration from numerous researchers around the world [16]. Silica (silicon dioxide) is the most bountiful mineral on the Earth [17]. Silica can be utilized as filler for rubber [18], which is utilized in the manufacture of numerous products, like tires and other industrial materials. Its numerous benefits include: improving the mechanical durability, shrinkage, heat resistance, thermal extension, and stress of the rubber-established composite. These merits are owing to the substitution of a soft matrix by hard inorganic filler. The silica was improved with silane coupling agents, preceding its application as reinforcement filler for the NR/SBR blend. The alteration prompted the improvement of the mechanical properties and thermal degradation. Composites dependent on the silica and modified-silica filled rubber [19], have been broadly studied.
Fly ash is unavoidably created as waste after burning. About 750 million tons of fly ash is produced yearly, nonetheless, just 39 % can be reused in the U.S [20]. If fly ash is discharged to the environment via air and wind, a severe contamination problem would be made with the hazard of pneumonic diseases. Recycling fly ash is an effective method to avoid such contamination. Comparable carbon black (CB) and precipitated silica, fly ash can likewise go about as reinforcing filler to enhance the mechanical properties of rubber compounds. Palmconstructed fly ash can be utilized to modify the mechanical properties of thermoplastic materials [21]. Glass waste that got from the metropolitan district can use as a brick added substance [22].
Everywhere in the world, the issue of disposing of waste tires is increasing day by day due to the growth in the number of vehicles on the road. The burning of tires and direct landfilling strategies prompts environmental degradation. Because of the non-biodegradable nature, the decomposition of wastes in the environment takes much time [23]. Upon manufacturing polymer blends or composites, gamma irradiation can be utilized to improve the compatibility between the ingredients by achieving the polymerization method. Besides, better interaction initiates with enhanced interfacial adhesion between composite components at low irradiation doses promoting improved properties without degrading the polymeric network [24].
In this study, we prepared styrene-butadiene rubber (SBR)/devulcanized waste tire as a rubber base for preparing composites depend on different silicate types and afterward exposed them to electron beam irradiation to induce crosslinking.
-TMTD, a reclaiming agent of the molecular formula C 6 H 12 N 2 S 4 , molecular weight 240.43 g/mol and density 1.43 g/cm 3 , provided by Zhedong Rubber Auxiliary Co., Ltd., China.
-Silicate additives are four different types of silica-containing fillers additives. Waste-Glass (WG), commercial soda-lime-silica glass, was utilized with the chemical compositions of WG particles used in this study are listed in Table I.
Silica bypass form ferromanganese byproduct named, Micaosilica (MS), that provided from Egyptian Ferroalloys Co., Egypt) has the next chemical composition presented in Table 1 Fly Ash (FA), having a density of 2.33 g/cm 3 and total evaporable moisture content of 1.54 % was formed from the ignition of coal as a byproduct. The particle size of FA falls in the range of 63 μm. The chemical composition of the FA (utilized in this study) was characterized by Energy dispersed X-ray analysis (EDX) as presented in Table 1.

Preparation of rubber composites
The SBR/DWR blends at 50/50 wt. %, the method for preparation of these blends was presented in previous work [25], and this composition was taken in this study, according to superlative mechanical properties gained for this blend.
Composites of this blend and other additives were prepared in an open mill as stated in Table 2. Subsequent to mixing, the samples were hot-pressed at 160 °C under 10 MPa for 5 min into sheets of fitting thickness and size for examination.

Electron beam irradiation of prepared blends
Irradiation of samples was done by utilizing electron beam accelerator (Energy 3 MeV, power 90 kW, Beam current 30 mA, conveyer speed 16m/min (50HZ) and scan width variable up to 90 cm) at the National Center for Radiation Research and Technology (NCRRT), Cairo, Egypt. The rubber blends were exposed to electron beam irradiation at ambient conditions for 100kGy.
Note: this dose of radiation 100 kGy was acquired, as it is appropriate dose for good mechanical properties of SBR/DWR 50/50 wt. % blends [25].

Mechanical measurements
The tensile properties of the dumbbell-shaped samples were estimated by Where t o is the original thickness of the sample; t 1 is the thickness of the sample after removal from the clamp, and t s is the thickness of the spacer bar utilized. Abrasion resistance tests were made by utilizing an abrasion tester type AP.40 (Maschinebau GmbH Rauenste in Thuringen, Germany). The loss in the mass percent was determined by ASTM D 3389-75 (1982), by equation (2): Where W i is the initial mass of sample (g), W f is the final mass of sample (g), and n is the number of revolutions, 84 revolutions.

Determination of cross-link density of rubber networks determined from
mechanical data Measurement of stress-strain response gives a simple technique for assessment of the cross-link density of polymer networks. As indicated by the classical kinetic theory of rubber elasticity was initially developed by Wall, Flory, James and Guth [26]. They ascribed the high elasticity of a cross-linked rubber to the variation of the conformational entropy of long flexible molecular chains. The theory predicts the following relation in simple extension; equation 3: Where σ is the true stress, the force per unit area measured in the strained state, υ e is the number of effective plastic chains per unit volume, K is Boltzmann's constant, T is the absolute temperature, and λ is the extension ratio; A Φ is a prefactor depending on the considered model. The elasticity of natural and SBR rubbers in simple extension at a constant strain rate was studied [27]. They plotted the true stress as a function of (λ 2 -λ -1 ) as recommended by the molecular theory.
They acquired a series of straight lines which don't go through the origin. In contrast, rubber elasticity theory predicts that the relation between the tensile strength and the elongation ratio, λ, is illustrated in equation 4: = ( ) + ( 2 − −1 ) (4) Where E is the modulus of elasticity and λ is the extension ratio of the strain that happened because of the applied stress. The relation between (λ 2 -λ -1 ) and stress (σ) for rubber blends or composites were drawn in figures. From these figures, it has been calculated the slope of the lines represent these relations, and then tried to calculate the average molecular weight Mc between cross-links from the value (G) as shown in equation 5: Where G is the shear modulus, ρ is the density of the rubber and R the gas constant, the value of M c , the molecular weight between two cross-links can be determined and then the cross-link density υ (CD) can be determined from equation 6: (υ) = 1/2 Thus cross-link density is contrarily proportioned with double the molecular weight between two cross-links. Consequently, cross-link density was established to be directly proportional to the true tensile modulus as indicated by equations 5 and 6.

FTIR investigation.
The chemical structure of the various types of silica untreated and treated with silane was analyzed by ATR-FTIR spectroscopy, Bruker Optik GmbH, Ettlingen, Germany, over the wavenumbers range 4000-500 cm -1 .

3-Results and discussion 3.1. Fourier Transform Infrared Spectrometry (FTIR)
FTIR is utilized to study the chemical interaction between silane and silica particles. Figure 1a shows a typical FTIR spectrum for micaosilica (SiO 2~9 0%) and its silane treatment. From the spectrum, the bands at 797 and 477 cm -1 are the characteristic bands of amorphous silica [28]. The strong broad band at approximately 1100 cm -1 corresponds to the frequency range of the siloxane group (Si-O-Si) vibration in silica [29]. The characteristic band located at 1620-1650 cm -1 was assigned to bending vibrations of aliphatic amine (N-H) groups. The surface of silica will usually contain an appreciable concentration of hydroxyl groups (-OH), the broad absorption band at 3300 -3730 cm -1 is due to the stretching vibration silanol groups (Si-OH), hydroxyl groups (water or alcohol) stretching by hydrogen bonding [30]. When the MS particles are treated silane, a new two bands appeared at 2853 and 2923 cm -1 characteristic to asymmetric and symmetric stretching vibrations of -CH 2 indicating grafting of APTMS on the surface of MS [31], the intensity of OH peak at 3300 -3730 cm -1 due to hydrogen bonding, besides stretching vibrations of aliphatic amine (N-H) groups for silane was not affected, despite, the involvement of the OH groups during the reaction of silane with silanol group of silica, as shown in figure 2 [31]. Furthermore, for the fly ash (SiO 2~5 9.23%), In raw fly ash (FA), the broad peak at 3412 cm -1 was ascribed to the stretching and deformation vibrations of OH and H-O-H groups from the water molecules, the presence of silica and alumina induced different linkages which have different vibration modes for identification. High intensity of Si-O-Si at 1435 cm -1 and Si-O-Al at 870 cm -1 [32] as shown in figure 1b. FA treated by silane shows the change in the intensity of the OH groups in silica, which resulted in lower reactivity of FA for silane than in the case of MS. FTIR spectra GW (SiO 2~7 1.9%) untreated with silane showed the same characteristic peaks of silica and after treated with silane the characteristic peak of silane is present with little change in the intensity of the OH groups in silica, which resulted in a lower quantity of hydrogen bond than in the case of MS as shown in Figure 1c.    Figure 3 represents the tensile strength (TS) for SBR/DWR 50/50 composites that unirradiated and irradiated at 100 kGy, also loaded with different silicate fillers with fixed concentrations namely, 40 phr. For unirradiated composites, the TS increases by addition silicate fillers; whatever it is, when compared with control samples; not contain fillers, and composites loaded by PS-MS 20/20 provided the highest values for TS, followed by PS, PS-MS 20/20, and finally PS-GW 20/20. The noticeable increase in TS of composites by PS-MS, due to MS contain high content of SiO 2 more than 90%, and by treated it by silane, it adhered by creating chemical bonds by silanol groups on the surface of silica, in addition to forming hydrogen bonds between rubber matrix and PS. Alternatively, irradiated composites provided the highest values for TS, when compared with unirradiated ones for the same silicate filler, and irradiated SBR/DWR 50/50 composites that loaded with PS-MS 20/20 gave the highest values for TS as discussed before, as well as increasing cross-link density caused by electron beam radiation that formed free radicals and produces link macromolecules of rubber with each other form one hand, and like of macroradicals of rubber with filler on the other hand. It is observed that irradiated composites loaded with PS-GW 20/20, PS, and PS-FA 20/20 gave comparable values for TS, due to the lower content of silica in it, when compared with MS. Also the increases of TS by radiation due to physical bonds formed between rubber matrix and silicate fillers by radiation.  Figure 4 exhibits the E b % for SBR/DWR 50/50 composites that unirradiated and irradiated at 100 kGy, also loaded with different silicate fillers with fixed concentration namely, 40 phr. Generally, for unirradiated composites, the E b decreases with the addition of silicate fillers, due to the filler restrict stretching. On the other hand, irradiated composites i.e blank samples, and also loaded ones, the E b increases by radiation for the same composites. For blank samples increases E b by radiation owing to liberation of carbon black in DWR which resulted in a decrease in E b . On the other hand, the E b increases by radiation for all silicate fillers owing to cross-linking not enough to retard elongation due to SBR rubber contain phenyl ring that dissipates the energy of electron beam irradiation, besides, it has low G(X) (cross-link/100 eV) about 0.3 when compared with other rubbers [33].   Figure 6 represents the variation of the cross-link density (CD) for SBR/DWR 50/50 composites which are unirradiated and irradiated at 100 kGy, also loaded with various silicate fillers with fixed concentration namely, 40 phr. Generally, the CD in unirradiated composites increases with the addition of all types of silicate fillers owing to interfacial adhesion between fillers and rubber matrix. Alternatively, irradiated and loaded composites presented comparable values for CD for all forms of silicate fillers, but MS remains has the highest value for CD owing to increasing interfacial adhesion and reactivity of this silicate filler.  Figure 7 elaborates the compression set for SBR/DWR 50/50 composites that unirradiated and irradiated at 100 kGy, also loaded with different silicate fillers with fixed concentration namely, 40 phr. For unirradiated composites, it can be observed that the blank SBR/DWR 50/50 samples have the highest compression set when compared with filled composite. At the same time, this performance was acquired in irradiated composites, but the values of compression set were decreased for the same type of silicate filler when compared with unirradiated one, owing to increased cross-link density which inversely proportional to cross-link density [ 34,35]. Besides, it can be seen that the SBR/DWR 50/50 composites loaded by PS-MS 20/20 gave the lowest values for compression set, and this was achieved for unirradiated and irradiated ones, owing to the increased cross-link density of these composites.  Figure 8 illustrates the hardness for SBR/DWR 50/50 composites that unirradiated and irradiated at 100 kGy, besides loaded with different silicate fillers with fixed concentration namely, 40 phr. For unirradiated composites, it can be observed that the blank SBR/DWR 50/50 samples have the lowest values for hardness for all unirradiated and irradiated composites; meanwhile, SBR/DWR 50/50 composites that loaded by PS-MS 20/20 provided the highest values for unirradiated and irradiated composites, owing to increased cross-link density. The composites loaded by PS, PS-MS 20/20, and PS-WG 20/20 provided comparable values for hardness and these for not irradiated composites and irradiated at 100 kGy.  Figure 9 demonstrates the abrasion loss for SBR/DWR 50/50 composites that unirradiated and irradiated at 100 kGy, also loaded with various silicate fillers with fixed concentration namely, 40 phr. It can be observed that, for unirradiated and irradiated SBR/DWR 50/50 composites, the abrasion resistance improved as follows; PS-MS 20/20> PS, PS-FA 20/20> PS-WG 20/20 > blank. These achieved results for abrasion confirmed by data discussed before in mechanical properties and cross-link density. Depending on these results of abrasion resistance, the SBR/DWR 50/50 composites loaded with silicate type PS-MS 20/20 were applicable in flooring tiles. The properties of obtained materials have been compared with commercially available floor tiles, as presented in Table 3. It can be seen that, SBR/DWR 50/50 composites loaded with silicate type PS-MS 20/20 at irradiated at 100 kGy were applicable in flooring tiles, the composites gave good mechanical properties when compared with others of commercially available.